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Graph of car fuel consumption versus speed



 
 
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  #1  
Old October 8th 08, 09:34 AM posted to uk.d-i-y
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Posts: 40
Default 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
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  #2  
Old October 8th 08, 09:58 AM posted to uk.d-i-y
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Posts: 2,911
Default 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.
  #3  
Old October 8th 08, 10:00 AM posted to uk.d-i-y
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Posts: 49
Default 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
  #4  
Old October 8th 08, 10:03 AM posted to uk.d-i-y
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Posts: 49
Default 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
  #5  
Old October 8th 08, 10:16 AM posted to uk.d-i-y
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Posts: 49
Default 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
  #6  
Old October 8th 08, 10:28 AM posted to uk.d-i-y
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Posts: 9,056
Default 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.
  #7  
Old October 8th 08, 10:34 AM posted to uk.d-i-y
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Posts: 2,911
Default 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.
  #8  
Old October 8th 08, 10:35 AM posted to uk.d-i-y
external usenet poster
 
Posts: 2,911
Default 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.
  #9  
Old October 8th 08, 10:53 AM posted to uk.d-i-y
external usenet poster
 
Posts: 9,377
Default 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?

  #10  
Old October 8th 08, 11:11 AM posted to uk.d-i-y
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Posts: 6,559
Default 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


 




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