Graph of car fuel consumption versus speed
 

I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.
--
Chris

Graph of car fuel consumption versus speed
 

Chris ] gurgled happily, sounding much like they were
saying:

I would like to make a graph of car fuel consumption versus speed. My
driving is not smooth enough to gather the raw data myself. Do you know
of any reliable figures, or graphs?

They don't exist.

I'm interested in relative values, rather than those for any particular
car.

They'll vary widely between classes of car - although individual cars
within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Graph of car fuel consumption versus speed
 

On Oct 8, 2:34*pm, Chris ] wrote:
Do you know of any reliable figures, or graphs?

Googling your subject gives a load of hits. Here's the first one:

http://metrompg.com/posts/speed-vs-mpg.htm

--
Nige Danton

Graph of car fuel consumption versus speed
 

On Oct 8, 2:58*pm, Adrian wrote:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds
lower than 60 mph.

--
Nige Danton

Graph of car fuel consumption versus speed
 

On Oct 8, 3:03*pm, Nige Danton wrote:
On Oct 8, 2:58*pm, Adrian wrote:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds
lower than 60 mph.

My response would have been better phrased as "drag cubes with
velocity; are cars really so slippery that drag does not become
significant until 60mph?"

--
Nige Danton

Graph of car fuel consumption versus speed
 

Chris wrote:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.

I think that frictional losses including the rolling resistance are
pretty much linear with speed, but aerodynamic drag is the cube of
velocity. Or it might be the square.

Hence economy driving broadly falls into these categories

To reduce frictional losses - otherwise approximately constant per mile
- lighten the car and pump the tyres up. And fit eco-tyres. You can
probably get 3-4% this way

- keep speeds below 60 mph at which point aero losses start to mount
sharply. This is significant. On cars with consumption meters 50-70
represents about 10% increase in fuel consumption, over that it goes up
massively.

- strip all external junk like roof racks and the like. There is
probably at 70mph a couple of percent to be had here.

- try and drive at a gear and speed where the engine is most efficient.
For a diesel that is at the lowest throttle setting IIRC where the
fuel-air ration is leanest. That possibly means use revs and less welly
to get acceleration and power, not slogging in a low gear at higher
throttle settings. For a petrol it may well be the other way around I am
not sure. This can net you about 5% from typical driving styles.

- reduce acceleration and braking to a minimum by anticipating the road.
Braking represents a net loss of energy that is never recoverable. This
is as great a contributions as speed reduction. Especially in towns.

Graph of car fuel consumption versus speed
 

The Natural Philosopher gurgled happily, sounding much like they
were saying:

On cars with consumption meters 50-70
represents about 10% increase in fuel consumption

Not always. You're ignoring the effect of gearing and the efficient rev
range of an engine.

Graph of car fuel consumption versus speed
 

Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds lower
than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

Graph of car fuel consumption versus speed
 

"Adrian" wrote in message
...
Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds lower
than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

So why do large commercials use lots of fuel at a constant 30, more at a
constant 40 and even more at a constant 50?
If drag isn't the cause what is?

Graph of car fuel consumption versus speed
 

"The Natural Philosopher" wrote in message
...
Chris wrote:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.

I think that frictional losses including the rolling resistance are pretty
much linear with speed, but aerodynamic drag is the cube of velocity. Or
it might be the square.

Hence economy driving broadly falls into these categories

To reduce frictional losses - otherwise approximately constant per mile -
lighten the car and pump the tyres up. And fit eco-tyres. You can probably
get 3-4% this way

- keep speeds below 60 mph at which point aero losses start to mount
sharply. This is significant. On cars with consumption meters 50-70
represents about 10% increase in fuel consumption, over that it goes up
massively.

- strip all external junk like roof racks and the like. There is probably
at 70mph a couple of percent to be had here.

- try and drive at a gear and speed where the engine is most efficient.
For a diesel that is at the lowest throttle setting IIRC where the
fuel-air ration is leanest. That possibly means use revs and less welly to
get acceleration and power, not slogging in a low gear at higher throttle
settings. For a petrol it may well be the other way around I am not sure.
This can net you about 5% from typical driving styles.

- reduce acceleration and braking to a minimum by anticipating the road.
Braking represents a net loss of energy that is never recoverable. This is
as great a contributions as speed reduction. Especially in towns.

There's been a lot about this on the radio in recent months, with people
wanting to cut their fuel consumption because of the price of it (now that
oil is back to $85 a barrel, why is petrol still £1.09 at the pumps?) and
the consensus is that the greatest savings to be had are by using gentle
acceleration. Fair enough. Unfortunately, a lot of people seem to have heard
this, and have taken it to heart without any thought. When joining a
motorway, or dual carriageway, your boot should be on the floor, especially
where it is an uphill slip road. You need to get your vehicle up to at least
the speed of traffic on the inside lane, so that you can make the judgement
to slip in behind or in front of any vehicle near you in that lane, without
causing any problem to them.

More and more people now seem to gently meander up the slip, without a
thought for anyone other than themselves, and then expect to be able to just
join the main carriageway, whilst everyone else takes care of letting them.
It has reached the point where I have almost been brought to a stop on the
slip by these thoughtless people, so many times in recent months, that I
groan whenever I swing off a roundabout, and see another motor chugging up
or down the slip, in front of me. Careful smooth driving ? Fine. But think
of the implications to others around you, when carrying out this
'eco-driving'.

Arfa

Graph of car fuel consumption versus speed
 

"dennis@home" gurgled happily, sounding
much like they were saying:

Drag cubes with velocity and so it may become important at speeds
lower than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

So why do large commercials use lots of fuel at a constant 30, more at a
constant 40 and even more at a constant 50? If drag isn't the cause what
is?

I'm not a truck driver, so I don't know if that's the case or not. If it
was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.

Graph of car fuel consumption versus speed
 

Adrian wrote:

I'm not a truck driver, so I don't know if that's the case or not. If it
was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.

yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).

Graph of car fuel consumption versus speed
 

In an earlier contribution to this discussion,
The Natural Philosopher wrote:


aerodynamic drag is the cube of
velocity. Or it might be the square.


Either - it depends on what parameter you're talking about!

Aerodynamic *force* is proportional to the square of velocity. The *power*
(force x speed) needed to overcome aerodynamic drag is proportional to the
cube of velocity.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Graph of car fuel consumption versus speed
 

Arfa Daily wrote:
"The Natural Philosopher" wrote in message
...
Chris wrote:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.
I think that frictional losses including the rolling resistance are pretty
much linear with speed, but aerodynamic drag is the cube of velocity. Or
it might be the square.

Hence economy driving broadly falls into these categories

To reduce frictional losses - otherwise approximately constant per mile -
lighten the car and pump the tyres up. And fit eco-tyres. You can probably
get 3-4% this way

- keep speeds below 60 mph at which point aero losses start to mount
sharply. This is significant. On cars with consumption meters 50-70
represents about 10% increase in fuel consumption, over that it goes up
massively.

- strip all external junk like roof racks and the like. There is probably
at 70mph a couple of percent to be had here.

- try and drive at a gear and speed where the engine is most efficient.
For a diesel that is at the lowest throttle setting IIRC where the
fuel-air ration is leanest. That possibly means use revs and less welly to
get acceleration and power, not slogging in a low gear at higher throttle
settings. For a petrol it may well be the other way around I am not sure.
This can net you about 5% from typical driving styles.

- reduce acceleration and braking to a minimum by anticipating the road.
Braking represents a net loss of energy that is never recoverable. This is
as great a contributions as speed reduction. Especially in towns.

There's been a lot about this on the radio in recent months, with people
wanting to cut their fuel consumption because of the price of it (now that
oil is back to $85 a barrel, why is petrol still £1.09 at the pumps?) and
the consensus is that the greatest savings to be had are by using gentle
acceleration. Fair enough. Unfortunately, a lot of people seem to have heard
this, and have taken it to heart without any thought. When joining a
motorway, or dual carriageway, your boot should be on the floor, especially
where it is an uphill slip road. You need to get your vehicle up to at least
the speed of traffic on the inside lane, so that you can make the judgement
to slip in behind or in front of any vehicle near you in that lane, without
causing any problem to them.


I m not convinced that slow acceleration is as effective as its made out
to be.

the energy needed to get a car up to speed is the same. Its just whether
the engine is operting more efficiently at high or low power outputs.

Now very high power outputs ARE inefficient, that's without doubt. But
whether 'tickover plus one' is more efficient than - say - half throttle
- is a really moot point.

We know, that at idle, producing no actual acceleration, the powertrain
is necessarily 0% efficient.

WE suspect that, at full power, its less efficient than part power. So
the curve of efficiency is definitely sort of parabolic. Where IS the
most efficient part?


Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.


My gut feel is that a petrol engine does best at about half RPM and half
throttle. A diesel possibly at somewhat higher RPM and somewhat less
throttle

Graph of car fuel consumption versus speed
 

Andy Burns wrote:
Adrian wrote:

I'm not a truck driver, so I don't know if that's the case or not. If
it was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.

yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).

50mph on single carriageways....

Graph of car fuel consumption versus speed
 

In article ],
Chris ] writes:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.

I've been playing with this, having recently got my first car
which displays fuel consumption. It wasn't quite what I expected.
Best fuel consumption is flat between 50MPH and 65MPH. Between
45MPH and 70MPH, efficiency doesn't drop off much. Below 45MPH,
efficiency drops off significantly (even when you can comfortably
stay in 5th gear, although I'm not sure it's making any difference
if I stay in 4th instead at lower speeds). Urban driving (start/stop
and never getting above 30MPH) is the worst of all.

Before I actually measured it, I would have guessed highest
efficiency would have been at lower speeds than it is. (The effect
of start/stop driving is no surprise of course.) I suspect engine,
gearbox, etc has probably been designed for good fuel consumption
at higher speeds instead, and this is more than countering increased
drag up to 65MPH.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Graph of car fuel consumption versus speed
 

The Natural Philosopher gurgled happily, sounding much like they
were saying:

I'm not a truck driver, so I don't know if that's the case or not. If
it was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.

yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).

Is that for efficiency reasons or for PR to try to cut back on the
"dualling artics"? I suspect the latter - because it would be FAR more
widespread, especially amongst smaller/pikier outfits - whilst the ones
who seem to do that seem to be the more "polished".

50mph on single carriageways....

Nope... 40mph. 50mph is only for 7.5ton.

Graph of car fuel consumption versus speed
 

Roger Mills wrote:
In an earlier contribution to this discussion,
The Natural Philosopher wrote:

aerodynamic drag is the cube of
velocity. Or it might be the square.


Either - it depends on what parameter you're talking about!

Aerodynamic *force* is proportional to the square of velocity. The *power*
(force x speed) needed to overcome aerodynamic drag is proportional to the
cube of velocity.

Right, So on a per mile basis, the work done is the force times the
distance? So in theory the *consumption* is affected as the square of
the velocity?

If you lump the two things together, you get that the frictional losses
are constant..so there is a fixed amount of gallons per mile you need
to burn just to keep moving irrsepective of speed, and the only way to
reduce that is with skinnier tyres and smaller cars and engines, and
pumping the tyres up harder..and a part which is related to the velocity
squared, which starts at zero, and increase accordingly.

So given a *uniformly efficient* powertrain, in cruise the slower you go
the less fuel you burn. Although the effects of aerodynamic drag are not
that great up to 50mph or so.

The next point is where are engines most efficient?

Graph of car fuel consumption versus speed
 

On Oct 8, 3:35*pm, Adrian wrote:
Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.
Drag cubes with velocity and so it may become important at speeds lower
than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

I'm a cyclist and aerodynamic friction plays a huge role in
determining speed and above ~30kph the benefits of drafting behind
another cyclist are considerable. There's an energy saving of ~20% for
the first cyclist in a pace line and that rises to maximum of ~30% for
the fourth cyclist.

I really would be surprised in cars are so slippery that aerodynamic
friction does not play a significant role at speeds slower than 60mph.

What sort of vehicles are you referring to?

--
Nige Danton

Graph of car fuel consumption versus speed
 

On Oct 8, 3:28*pm, The Natural Philosopher wrote:

I think that frictional losses including the rolling resistance are
pretty much linear with speed, but aerodynamic drag is the cube of
velocity. Or it might be the square.

Aerodynamic drag cubes with speed.

--
Nige Danton

Graph of car fuel consumption versus speed
 

Andrew Gabriel wrote:
In article ],
Chris ] writes:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.

I've been playing with this, having recently got my first car
which displays fuel consumption. It wasn't quite what I expected.
Best fuel consumption is flat between 50MPH and 65MPH. Between
45MPH and 70MPH, efficiency doesn't drop off much. Below 45MPH,
efficiency drops off significantly (even when you can comfortably
stay in 5th gear, although I'm not sure it's making any difference
if I stay in 4th instead at lower speeds). Urban driving (start/stop
and never getting above 30MPH) is the worst of all.

Before I actually measured it, I would have guessed highest
efficiency would have been at lower speeds than it is. (The effect
of start/stop driving is no surprise of course.) I suspect engine,
gearbox, etc has probably been designed for good fuel consumption
at higher speeds instead, and this is more than countering increased
drag up to 65MPH.


I think its more to do with having to meet targets set at 56mph speeds..

You can in theory optimize design for any speed..and I suspect thats the
one thats chosen. I.e. they have set the motor to be geared optimally
for 56mph on a flat road..

I used to notice similar on the last car I had with a consumption meter.
There was little to be gained sub 50mph.

My camper seems to be relatively unaffected whether I drive at 50 or 60
either, but does show a marked worsening at 70mph.

I am scratching my memory for where engines are efficient..I am fairly
sure with a diesel its actually lightly loaded but at reasonably high
RPM..the friction loses in a slow revving diesel are low anyway, so you
can go for the weakest mixture and the most complete combustion at the
highest temp. Thats at lowish throttle settings.

For petrol cars, with injection, I am less sure where they are optimal.
It is certainly wasteful to rev to the limit. But whether flat out at
1500 RPM or half pedal at 3000 is better or worse I couldn't say.

You might even find that periods of acceleration and periods of coasting
is in fact a better way to drive.

Graph of car fuel consumption versus speed
 

Nige Danton gurgled happily, sounding much like
they were saying:

I really would be surprised in cars are so slippery that aerodynamic
friction does not play a significant role at speeds slower than 60mph.

What sort of vehicles are you referring to?

There's a clue in my email address...

Graph of car fuel consumption versus speed
 

On Oct 8, 9:53*am, "dennis@home"
wrote:
"Adrian" wrote in message

...

Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds lower
than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

So why do large commercials use lots of fuel at a constant 30, more at a
constant 40 and even more at a constant 50?
If drag isn't the cause what is?

It's all to do with the trans-dimensional (in)stability of the Yorkie
Bar.

MBQ

Graph of car fuel consumption versus speed
 

AJH wrote:
On 08 Oct 2008 10:25:39 GMT, (Andrew
Gabriel) wrote:

I suspect engine,
gearbox, etc has probably been designed for good fuel consumption
at higher speeds instead, and this is more than countering increased
drag up to 65MPH.

I'm sure you are right, the engine is probably the first thing
optimised for this speed, below the optimum the thermodynamic
efficiency tails off significantly with a petrol engine, less so with
a diesel.

A smaller engine in the same car should move this part of the graph
toward higher mpg. I think this is one of the reasons the original
hybrids appeared poor in open road conditions they had smaller engines
and did best at about 45mph on the open road.

With an average drag I think the rolling resistance equals the wind
resistance at less than 20mph (12mph for a cyclist) and intuitively
this point is the best combination of energy efficiency and
timeliness.


Highly debatable.

Light skinny cars with skinny wheels and small engines probably peak
there, but heavier cars with fatter tyres and bigger engines probably
peak - albeit at far worse consumption figures - somewhere in the 50-60
mph mark.



AJH

Graph of car fuel consumption versus speed
 

On Oct 8, 9:16 am, Nige Danton wrote:
On Oct 8, 3:03 pm, Nige Danton wrote:

On Oct 8, 2:58 pm, Adrian wrote:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds
lower than 60 mph.

My response would have been better phrased as "drag cubes with
velocity; are cars really so slippery that drag does not become
significant until 60mph?"

I *think* that the force exerted by drag goes as the /square/ of the
speed. That means the energy (and hence fuel consumption) per unit
distance will also go as the square. I suspect what you are thinking
of is that the /power/ (energy per unit time) is force * distance /
time which is force * speed, and hence the power used by drag goes as
the cube of speed.

Graph of car fuel consumption versus speed
 

On Oct 8, 11:30 am, Adrian wrote:
The Natural Philosopher gurgled happily, sounding much like they
were saying:

I'm not a truck driver, so I don't know if that's the case or not. If
it was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.
yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).

Is that for efficiency reasons or for PR to try to cut back on the
"dualling artics"? I suspect the latter - because it would be FAR more
widespread, especially amongst smaller/pikier outfits - whilst the ones
who seem to do that seem to be the more "polished".

The optimum speed for heavy duty trucks depends on quite a number of
things:
- fuel consumption
- using the (expensive) truck as much as possible
- urgency of the cargo
- reaching the destination before the driver has to take a break.

Certainly some North American fleets limit their trucks to below the
legal maximum to improve fuel economy.

Graph of car fuel consumption versus speed
 

Martin Bonner wrote:
On Oct 8, 9:16 am, Nige Danton wrote:
On Oct 8, 3:03 pm, Nige Danton wrote:

On Oct 8, 2:58 pm, Adrian wrote:
within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.
Drag cubes with velocity and so it may become important at speeds
lower than 60 mph.
My response would have been better phrased as "drag cubes with
velocity; are cars really so slippery that drag does not become
significant until 60mph?"

I *think* that the force exerted by drag goes as the /square/ of the
speed. That means the energy (and hence fuel consumption) per unit
distance will also go as the square. I suspect what you are thinking
of is that the /power/ (energy per unit time) is force * distance /
time which is force * speed, and hence the power used by drag goes as
the cube of speed.

I think we all agree. I have to often estimate drag in power terms in my
hobby*..and thats where the confusion came in.


* model aircraft,where someone always wants one that goes faster..which
ultimately becomes how much power you can cram in without the thing
needing a catapult or a mile of runway to get off the ground. Getting
beyond 200mph is pretty hard for a powered one (though jets are up
there), though the current unpowered speed record is about 300mph I
think. Don't ask how...google 'dynamic soaring' and take out a loan with
Northern Rock if interested.

Graph of car fuel consumption versus speed
 

Martin Bonner wrote:
On Oct 8, 11:30 am, Adrian wrote:
The Natural Philosopher gurgled happily, sounding much like they
were saying:

I'm not a truck driver, so I don't know if that's the case or not. If
it was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.
yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).
Is that for efficiency reasons or for PR to try to cut back on the
"dualling artics"? I suspect the latter - because it would be FAR more
widespread, especially amongst smaller/pikier outfits - whilst the ones
who seem to do that seem to be the more "polished".

The optimum speed for heavy duty trucks depends on quite a number of
things:
- fuel consumption
- using the (expensive) truck as much as possible
- urgency of the cargo
- reaching the destination before the driver has to take a break.


Add stability and safety to that. Very heavy loads on an articulated
truck are not things to stop in a hurry.


Certainly some North American fleets limit their trucks to below the
legal maximum to improve fuel economy.

And thats generally why the convoy each other as well. Up to 25% less
drag in a 'train'

Graph of car fuel consumption versus speed
 

"The Natural Philosopher" wrote in message
...
AJH wrote:
On 08 Oct 2008 10:25:39 GMT, (Andrew
Gabriel) wrote:

I suspect engine,
gearbox, etc has probably been designed for good fuel consumption
at higher speeds instead, and this is more than countering increased
drag up to 65MPH.

I'm sure you are right, the engine is probably the first thing
optimised for this speed, below the optimum the thermodynamic
efficiency tails off significantly with a petrol engine, less so with
a diesel.

A smaller engine in the same car should move this part of the graph
toward higher mpg. I think this is one of the reasons the original
hybrids appeared poor in open road conditions they had smaller engines
and did best at about 45mph on the open road.

With an average drag I think the rolling resistance equals the wind
resistance at less than 20mph (12mph for a cyclist) and intuitively
this point is the best combination of energy efficiency and
timeliness.


Highly debatable.

Light skinny cars with skinny wheels and small engines probably peak
there, but heavier cars with fatter tyres and bigger engines probably
peak - albeit at far worse consumption figures - somewhere in the 50-60
mph mark.



AJH


This link might help in understanding the effects of wind drag
http://www.youtube.com/watch?v=lttgT1XZVvE
It's an episode of MythBusters, where they tested whether driving behind a
large truck saved fuel.

Very interesting results..

Mat

Graph of car fuel consumption versus speed
 

Mat C wrote:


"The Natural Philosopher" wrote in message
...
AJH wrote:
On 08 Oct 2008 10:25:39 GMT, (Andrew
Gabriel) wrote:

I suspect engine,
gearbox, etc has probably been designed for good fuel consumption
at higher speeds instead, and this is more than countering increased
drag up to 65MPH.

I'm sure you are right, the engine is probably the first thing
optimised for this speed, below the optimum the thermodynamic
efficiency tails off significantly with a petrol engine, less so with
a diesel.

A smaller engine in the same car should move this part of the graph
toward higher mpg. I think this is one of the reasons the original
hybrids appeared poor in open road conditions they had smaller engines
and did best at about 45mph on the open road.

With an average drag I think the rolling resistance equals the wind
resistance at less than 20mph (12mph for a cyclist) and intuitively
this point is the best combination of energy efficiency and
timeliness.


Highly debatable.

Light skinny cars with skinny wheels and small engines probably peak
there, but heavier cars with fatter tyres and bigger engines probably
peak - albeit at far worse consumption figures - somewhere in the
50-60 mph mark.



AJH


This link might help in understanding the effects of wind drag
http://www.youtube.com/watch?v=lttgT1XZVvE
It's an episode of MythBusters, where they tested whether driving behind
a large truck saved fuel.

Very interesting results..


A bit better than I expected, actually. but something every racing
driver knows,and anyone who has driven anything resembling a commercial
vehicle, and something that has been proven time and again in analysis
of train efficiencies.


Mat

Graph of car fuel consumption versus speed
 

"Chris" ] wrote in message
]...
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.

The equations and sample data are on my website.

http://www.pumaracing.co.uk/TOPSPEED.htm
--
Dave Baker

Graph of car fuel consumption versus speed
 

AJH wrote:
On Wed, 8 Oct 2008 14:20:16 +0100, "Mat C" wrote:

http://www.youtube.com/watch?v=lttgT1XZVvE
It's an episode of MythBusters, where they tested whether driving behind a
large truck saved fuel.

It makes a fuel efficiency case for allowing long distance trucks to
couple up!

AJH

It's called a railway...

Graph of car fuel consumption versus speed
 

AJH wrote:
On Wed, 8 Oct 2008 14:20:16 +0100, "Mat C" wrote:

http://www.youtube.com/watch?v=lttgT1XZVvE
It's an episode of MythBusters, where they tested whether driving behind a
large truck saved fuel.

It makes a fuel efficiency case for allowing long distance trucks to
couple up!

AJH


Start with two, and by the end of the journey you have the two big ones
and a little one as well?

Isn't this (coupling up) effectively what happens with the lorry and
dangler arrangements?

--
Rod

Hypothyroidism is a seriously debilitating condition with an insidious
onset.
Although common it frequently goes undiagnosed.
www.thyromind.info www.thyroiduk.org www.altsupportthyroid.org

Graph of car fuel consumption versus speed
 

Dave Baker wrote:
"Chris" ] wrote in message
]...
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.

The equations and sample data are on my website.

http://www.pumaracing.co.uk/TOPSPEED.htm


Excellent site Dave. And it shows that for 100mph about 70bhp/ton is
fairly close. and no more than 35 needed for 70mph.

However it does seem to be a bit conservative - many cars exceed 60mpg
at 56mph, and also the fact of the matter is that so long as the big
heavy cars aero losses are well below the frictional losses it doesn't
matter a damn what speed it goes at. I.e. my old Jaguar would never ever
turn in better than 27mpg no matter how slow it was driven. Typically it
did 19-20mg. To get 27mpgh required it to be trundled at - yas - 56mph
constantly. One stop would ruin it!



It does show that there is plenty of room fr improvement though. Low
friction small turbo charged engine, and lighter more aerodynamic cars
and skinnier tyres could still have the speed. motorcycle has the speed
but not the fuel consumption, after all.

Graph of car fuel consumption versus speed
 

The message
from The Natural Philosopher contains these words:

We know, that at idle, producing no actual acceleration, the powertrain
is necessarily 0% efficient.

WE suspect that, at full power, its less efficient than part power. So
the curve of efficiency is definitely sort of parabolic. Where IS the
most efficient part?


Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.

It is more than 40 years since I did any thermodynamics and we didn't do
much on IC engines anyway but through the mists of time ISTR that an IC
engine is most efficient at max bmep (brake mean effective pressure
IIRC) and max bmep equates very closely to max torque.

One of the reasons I remember this at all is because it seems to
conflict with the age old advice that the way to save fuel is to get
into as high a gear as possible as early as possible.

--
Roger Chapman

Graph of car fuel consumption versus speed
 

"Adrian" wrote in message
...
Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds lower
than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

Well it's present at any speed and where we determine it becomes significant
is actually a function of how large the drag force is compared to the
rolling resistance one which stays pretty much a constant at all speeds. For
an average modern streamlined but heavy car like my Focus the drag force is
fairly immaterial below 30 mph. At 45 mph the engine power being used to
overcome drag is about the same as that being used to overcome rolling
resistance. At 65 mph it's twice as much and at 80 mph three times as much.

However pick a light but less streamlined car like an old Fiesta or Golf
from the 80's and the picture changes. At 2000 lbs with driver rather than
the 3000 of the Focus but 15% to 20% higher drag the relationship is very
different.
The power being used to overcome drag is the same as that being used to
overcome RR by 35 mph. At 60 mph it's three times as much and at 80 mph five
times as much.

Light cars therefore benefit from going slower much more than heavy ones in
terms of fuel consumption. This is exacerbated by the fact that the large
engines of large heavy cars, as with all petrol engines, only work
efficiently at reasonable throttle openings and at low speed on a whisper of
throttle the cylinders are not filling well. A small light car with a small
engine is doing much better in all respects here. You want a diesel for good
efficiency at any power output.

What this translates into is my Focus doesn't really get much different fuel
consumption whether I do 60 mph or 80 mph. It seems to peak at about 38 mpg
and I just can't improve on that. However an old Fiesta XR2i I used to run
which gave 34 mpg on average once achieved 51.5 mpg when I stuck to 40 mph
on A roads and 50 mph on motorways during the fuel shortage in 2000. If I
gave the Focus the same treatment I doubt I'd beat 40 mpg by much. It's just
too heavy.

The formula for good fuel economy is very simple. Small, light, aerodynamic
cars with small low friction diesel engines. The 100 mpg practical car is a
very easy thing to design but we'll only see them when fuel prices bite even
harder. I've designed one in basic terms which would take me from London to
Aberdeen, which I do fairly regularly, for 1/3 the fuel cost of the Focus.
At present a round trip costs me £150. That's a big chunk of a holiday cost
when you can fly abroad with Sleasyjet for a tenner. At £50 it wouldn't be
much of a burden.
--
Dave Baker

Graph of car fuel consumption versus speed
 

gurgled happily, sounding much like they were saying:

The 'official' tests for fuel consumtion were for 56mph.

They haven't been urban/56/75 since the mid 1990s! The current urban/
extra-urban figures are done across a variety of speeds and loads,
including acceleration and deceleration.

Graph of car fuel consumption versus speed
 

"The Natural Philosopher" wrote in message
...
Arfa Daily wrote:
"The Natural Philosopher" wrote in message
...
Chris wrote:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.
I think that frictional losses including the rolling resistance are
pretty much linear with speed, but aerodynamic drag is the cube of
velocity. Or it might be the square.

Hence economy driving broadly falls into these categories

To reduce frictional losses - otherwise approximately constant per
mile - lighten the car and pump the tyres up. And fit eco-tyres. You can
probably get 3-4% this way

- keep speeds below 60 mph at which point aero losses start to mount
sharply. This is significant. On cars with consumption meters 50-70
represents about 10% increase in fuel consumption, over that it goes up
massively.

- strip all external junk like roof racks and the like. There is
probably at 70mph a couple of percent to be had here.

- try and drive at a gear and speed where the engine is most efficient.
For a diesel that is at the lowest throttle setting IIRC where the
fuel-air ration is leanest. That possibly means use revs and less welly
to get acceleration and power, not slogging in a low gear at higher
throttle settings. For a petrol it may well be the other way around I am
not sure. This can net you about 5% from typical driving styles.

- reduce acceleration and braking to a minimum by anticipating the road.
Braking represents a net loss of energy that is never recoverable. This
is as great a contributions as speed reduction. Especially in towns.

There's been a lot about this on the radio in recent months, with people
wanting to cut their fuel consumption because of the price of it (now
that oil is back to $85 a barrel, why is petrol still £1.09 at the
pumps?) and the consensus is that the greatest savings to be had are by
using gentle acceleration. Fair enough. Unfortunately, a lot of people
seem to have heard this, and have taken it to heart without any thought.
When joining a motorway, or dual carriageway, your boot should be on the
floor, especially where it is an uphill slip road. You need to get your
vehicle up to at least the speed of traffic on the inside lane, so that
you can make the judgement to slip in behind or in front of any vehicle
near you in that lane, without causing any problem to them.


I m not convinced that slow acceleration is as effective as its made out
to be.

the energy needed to get a car up to speed is the same. Its just whether
the engine is operting more efficiently at high or low power outputs.

Now very high power outputs ARE inefficient, that's without doubt. But
whether 'tickover plus one' is more efficient than - say - half throttle -
is a really moot point.

We know, that at idle, producing no actual acceleration, the powertrain is
necessarily 0% efficient.

WE suspect that, at full power, its less efficient than part power. So the
curve of efficiency is definitely sort of parabolic. Where IS the most
efficient part?


Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.

You get better MPG in higher gears which gives less acceleration.

You also get better mpg by using just enough gas to give the acceleration
needed, not the maximum acceleration you can get.
You need to be able to drive to know what acceleration is needed rather than
just putting your foot down and this is where most come unstuck.
I seldom need more than a quarter throttle to match motorway speeds on any
of the slip roads I use. I have had to use the hard shoulder when some prat
has decided to stop because he can't get into a gap. Using the hard shoulder
is the correct way of doing it of course.

Graph of car fuel consumption versus speed
 

"Roger" wrote in message
k...
The message
from The Natural Philosopher contains these words:

We know, that at idle, producing no actual acceleration, the powertrain
is necessarily 0% efficient.

WE suspect that, at full power, its less efficient than part power. So
the curve of efficiency is definitely sort of parabolic. Where IS the
most efficient part?


Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.

It is more than 40 years since I did any thermodynamics and we didn't do
much on IC engines anyway but through the mists of time ISTR that an IC
engine is most efficient at max bmep (brake mean effective pressure
IIRC) and max bmep equates very closely to max torque.

One of the reasons I remember this at all is because it seems to
conflict with the age old advice that the way to save fuel is to get
into as high a gear as possible as early as possible.

That depends on the torque curves and old long stroke engines had their
torque low down so it was true.
My wife's corsa has the torque low down since I had the engine map changed.
Its far better to have low end torque than a few extra bhp. Its pretty
stupid to have high bhp at the expense of torque with 70 mph speed limits.

--
Roger Chapman

Graph of car fuel consumption versus speed
 

On Oct 8, 3:22*pm, Roger wrote:
The message
from The Natural Philosopher contains these words:

We know, that at idle, producing no actual acceleration, the powertrain
is necessarily 0% efficient.
WE suspect that, at full power, its less efficient than part power. So
the curve of efficiency is definitely sort of parabolic. Where IS the
most efficient part?
Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.

It is more than 40 years since I did any thermodynamics and we didn't do
much on IC engines anyway but through the mists of time ISTR that an IC
engine is most efficient at max bmep (brake mean effective pressure
IIRC) and max bmep equates very closely to max torque.

That's what my father always said (max torque).

MBQ