What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On Sunday, July 23, 2017 at 2:59:04 PM UTC-4, dpb wrote:
On 07/23/2017 1:32 PM, wrote:
On Sun, 23 Jul 2017 11:21:22 -0500, wrote:
...

I didn't try to find what Canadian limits are -- I presume they must be
somewhat more stringent in order to match, more or less, the size of the
measurement interval? You know?

I do not know the requirements or test results today, but I DO know
back when we had mechanical meters our pumps (at the stations where I
worked)were never out by more than a couple oz in the 5 gallon
calibration, and the new electronic metering pumps (deployed when we
switched from imperial Gallons to Liters) were "significantly more
accurate" in metering. The accuracy changed a bit with delivery speed
on the mechanical pumps - can't remember if the change was that the
read higher or lower with reduced pump speed, but the variance was
quite low. The electronic meters were supposedly less susceptible to
volume arrors based on fuel velocity.
...

As noted, being they're dispensing quantities measured in units that are
roughly one-fourth the size one would suppose tolerances would be
adjusted similarly. The 0.5% error with the US compliance standard
would be almost 2% which seems as would be excessive in comparison to
what a governing consumer-protection function would deem adequate.

Nothing came up in the searches I did, but I didn't look for non-US
data, either, so being in US not surprising what I found was what I
found... :)

Anyways, I think we can put the subject to rest... :)

--

I don't see any reason that the accuracy of pumps that measure in
liters would be any different than the accuracy of pumps measuring
in gallons. The actual quantities dispensed are the same, you'd
think the pumps probably use very similar measuring technology,
maybe even exactly the same pumps, just that one reads out in
gallons, the other liters. Also, since the obvious requirement
standards for accuracy is so that the consumer doesn't get screwed
and the transaction is far, it would seem that the standards
would be set similarly. If the US standard is to be off by no
more than 6 cubic inches in 5 gallons, then I'd expect the EU
standard would be similar, eg 100 ml in 20 liters. That puts
in at ~ 0.5% accuracy which is reasonable for a consumer
transaction. I doubt milk, olive oil or anything else is
required to be any more accurate than that.

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On 07/23/2017 2:18 PM, trader_4 wrote:
....

I don't see any reason that the accuracy of pumps that measure in
liters would be any different than the accuracy of pumps measuring
in gallons. The actual quantities dispensed are the same, you'd
think the pumps probably use very similar measuring technology,
maybe even exactly the same pumps, just that one reads out in
gallons, the other liters. ...


That's actually probably the case; somewhere along the way I ended up
with a number from the NIST data that came out pretty close to 0.1 gal
and made a connection that's likely not there in reality. Now I don't
recall just where the number came from, even--and, it might've been a
misstep along the way, besides. :)

I do think it somewhat amusing that NIST is still using the '6 cu in/5
gal' paradigm given they're the "standards" folks. :) I didn't try to
research the history but I'm sure this is a historical artifact that
probably goes back to the '20s or '30s and it's just too ingrained to
change.

--

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On 07/23/2017 04:44 PM, dpb wrote:
I do think it somewhat amusing that NIST is still using the '6 cu in/5
gal' paradigm given they're the "standards" folks. :) I didn't try to
research the history but I'm sure this is a historical artifact that
probably goes back to the '20s or '30s and it's just too ingrained to
change.

What else would they use? The pumps are measuring volume so it's going
to be some volume and cubic inches is as good as anything. I would
assume they have some sort of necked container with go/no-go lines on
the neck.

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On 07/23/2017 9:07 PM, rbowman wrote:
On 07/23/2017 04:44 PM, dpb wrote:
I do think it somewhat amusing that NIST is still using the '6 cu in/5
gal' paradigm given they're the "standards" folks. :) I didn't try to
research the history but I'm sure this is a historical artifact that
probably goes back to the '20s or '30s and it's just too ingrained to
change.

What else would they use? The pumps are measuring volume so it's going
to be some volume and cubic inches is as good as anything. I would
assume they have some sort of necked container with go/no-go lines on
the neck.

Well, "Standards" are universally written in metric these days--English
units (and a mismatched set at that) is the anachronism...

--

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

rbowman wrote on 7/22/2017 1:38 PM:
On 07/22/2017 07:22 AM, rickman wrote:
rbowman wrote on 7/22/2017 1:29 AM:
On 07/21/2017 07:47 PM, wrote:
Occaisionally on a longish trip I'll see how well I can drive for
economy - to see if I can better the last time I did that trip.

I'm a fairly economical driver but on longish trips I'm more concerned
with
getting there. 80 mph guarantees the fuel economy is going into the
dumpster.

I forgot, I can tell the difference in fuel economy by driving 65 MPH
rather than 60. Driving at 65 very much (only about 1/3 of my trip
allows that) will assure that I only get 19 mpg rather than pushing 20.

There is a 10 mile stretch with only one traffic light and a posted
speed limit of 45 MPH. If I can get up to 50 so I'm solid in fifth gear
my mileage rocks.


I should look at the instantaneous readouts versus mph to see if the mpg
falls off gradually or if there is an efficiency sweet spot around 65-70.
Except for around the cities the interstate speed limit in this and some of
the adjoining states is 80. Drive 65 at your own risk.

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.

--

Rick C

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

Mad Roger wrote on 7/22/2017 7:42 PM:
On Sat, 22 Jul 2017 00:46:50 -0400,
rickman wrote:

So my odometer is accurate and precise.

I understand you because you're exactly the type of person that I had in
mind when I asked the question in the first place.

I don't know what you mean. I have checked my odometer against the markers
on the highway as well as against my GPS (I think the highway markers are
more accurate than the GPS). It is spot on with the current tires to 1% or
better.

Does your tripmeter have a decimal place and digits after that decimal
place?

I've never seen a trip odometer that didn't have tenths of a mile.


My speedometer is mechanical and so has a separate calibration factor.

The speedometer example was only brought in to point out that the vain hope
that averages result in better "accuracy" is patently false.

Only because averages don't impact the effect of limited accuracy, averaging
mitigates the effect of limited precision. But both precision and accuracy
impact the error in any one reading.


Mom-and-pop type of people actually believe that a speedometer reads even
close to accurately - and worse - some here propose the vain notion that
the more readings they take, somehow (magically?) the more accurate the
results will be.

A speedometer that reads high isn't going to result in more accurate
calculations even if you do a billion test runs.

+ A pumpmeter of 20.25 gallons is likely relatively accurate & precise

Of course it is. States inspect them at some point.

You don't seem to understand what accuracy and precision even mean.
Haven't you taken even one science lab course?

I think you are missing something. What you replied do does not in any way
indicate a limited understanding of precision and accuracy. But affect each
measurement taken. An inspection measurement will require the combination
of accuracy and precision in that measurement be within some limit. What do
you expect them to do, take dozens of measurements? There are economic
considerations, especially since this is about economics anyway. It is to
prevent excess profits from being made by shortchanging the customers.


+ Matching fuel level in the tank isn't even close to accurate nor precise

I don't agree. I let the pump click off and then continue to pump for a
number of more clicks until it cuts off immediately.

I'm not at all surprised about your concept of the fuel-level estimation,
and, in fact, you're exactly the mom-and-pop type person I was talking
about when I opened the thread.

I understand you.

Not sure what that means. What I am doing by repeatedly topping off is to
reach the point where the fuel in the filler neck is right at the nozzle so
it won't run anymore, but rather cuts off immediately. This results in a
very consistent fill level.


I always need to run
at least another fifteen miles before I am home so that is better part of a
gallon burned so I don't need to worry about the gas warming up and running
out of the tank. I believe this makes for very consistent fill ups.

I'm sure you do believe that.

I think my consistent mileage measurements support my conclusions.


My MPG results pretty well show the consistency of my measures.

I'm sure your MPG results support any theory you want them to support.
I believe you.

You seem to be doubting my results. Are you suggesting I am fudging my data?


You know what happens when you assume... ;)

You don't know how funny that statement was to me when I just read it now.

I see less than 19 or even 19.5 MPG.

I bet you see that decimal place even though it's not in the tripmeter
estimation nor in the filllevel estimation.

You seem obsessed with evaluating the resulting MPG measurement even though
you can't put numbers on the accuracy of the parameters that impact the MPG
errors. If you can't come up with numbers, your ideas are of no value. But
that doesn't mean the errors in my MPG measurements aren't as they appear to
be.

Actually, I do have numbers for the parameters. I know the mileage to a
fraction of a mile (even though a tenth mile out of 400 is far more accurate
than anything else involved) and I have no reason to doubt the pump giving
me 20.0 gal when it says 20.0 gal. I don't fill up at the same pump each
time so if some were off it would show up and I'd be able to identify which
pumps were inaccurate.


You see, I understand you because you're the type of person I had in mind
when I asked the question.

I think the consistency of my MPG readings show how well each of these can
be measured.

I'm sure you do.

You keep saying this without indicating what you mean.


As you say, the pump is going to be dead on.

Whoa! I never said the pump was "dead on" and anyone reading this thread
who thinks I think the pump is "dead on" would have completely
misunderstood everything else I said.

All I said was that the inaccuracies and imprecisions in the pump reading
are likely better than the otherwise astoundingly huge imprecision in the
fuel-fill level estimation and in the lesser inaccuracy of the tripmeter
estimation.

Lol! You see, I understand you because you're the type of person I had in
mind when I made that comment.


Other than scale
error which can be calibrated out the odometer will be very good.

Define "very good" please.

Have done, 0.1 mile over 100 miles has been calibrated... actually, it was
much better than 0.1 mile since I can interpolate the analog dial. I don't
drive that stretch of road anymore, so I can't calibrate 100.0 miles
continuously anymore or I would.


Filling your tank can be good as well.

I'm sure you believe that filling the tank is "accurate" since you
calculate 19.5 miles per gallon and not something like 19.5 rounded up to
20 and then the error taken into account such that it's more likely
anywhere between 19 and 21 mpg than it is 19.5 mpg.

Sorry, your sentence doesn't make sense to me. Can you construct it properly?


It's not like they design gas tanks to have air pockets.

Actually, they do have air pockets.
Those air pockets change in size based on temperature & pressure & fill
level.

Even the fuel changes in density based on those parameters.

You don't need to know any of this specifically.

Of course I don't. 19.5 mpg is all I need to know.
And if I change "something" which results in 19.7mpg, then of course, that
something was the cause. I understand. I really do.

Why do you care which of the three has what specific degrees of accuracy and
precision?

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.

If what you say is true, why is it I have only seen 21 mpg a very, very few
times in the 20 years I have been checking my mileage? If what you are
saying is true, I should see a much wider variation in measurements than I
see. As I have said, 95% of the time I get between 19.5 and 20.5 mpg or
within a 4% range (+-2%). It's actually even tighter than that. It's more
like 19.7 to 20.2 mpg but I can't say just how often.

--

Rick C

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

rickman wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)
 

Vic Smith posted for all of us...



I never used the tripmeter for MPG, because I never bothered testing them with mile
markers.
Matching gas level is trivial - and it only has to done at the beginning and end of the
trip.
Gas station pumps - I assume they are accurate, and can't control that anyway.
I'm confident that my measurements are accurate to within .1 MPG."

His response to me totally ignored those responses, and he posed the same questions again!
Then, for some reason, he stated talking about speedometers.
He's a troll.


Exactly what I have been posting. This guy is the valve stem thread, bead
breaker, etc troll.

--
Tekkie

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)
 

dpb posted for all of us...



On 07/23/2017 1:12 AM, rbowman wrote:
On 07/22/2017 10:45 PM, Bill Vanek wrote:
On Sat, 22 Jul 2017 22:12:26 -0600, rbowman
wrote:

...

That stupid speed limit is the least of Oregon's problems.

Where is their limit 55?


The last time I was there US20, US395, and other 2 lane roads in eastern
Oregon. Apparently the raised it to 65 in March of 2016 but are rolling
it back in some places.
...


They're not the only seemingly bizarre place--between Clayton and
Springer is 100 mi of open country with either 55 (or _maybe_ 60) that
makes no common sense at all...

Where does one find common sense in da govt?

--
Tekkie

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On 7/24/17 3:19 PM, Tekkie® wrote:
dpb posted for all of us...



On 07/23/2017 1:12 AM, rbowman wrote:
On 07/22/2017 10:45 PM, Bill Vanek wrote:
On Sat, 22 Jul 2017 22:12:26 -0600, rbowman
wrote:

...

That stupid speed limit is the least of Oregon's problems.

Where is their limit 55?


The last time I was there US20, US395, and other 2 lane roads in eastern
Oregon. Apparently the raised it to 65 in March of 2016 but are rolling
it back in some places.
...


They're not the only seemingly bizarre place--between Clayton and
Springer is 100 mi of open country with either 55 (or _maybe_ 60) that
makes no common sense at all...

Where does one find common sense in da govt?


Yeah, as @patsajak noted (and especially in uber-liberal states like
Oregon), politicians have learned that it's more fun to over-control
people and tell them how to live than it is to fix the potholes

--
The taxpayers are sending congressmen on expensive trips overseas. It
might be worth it, except they keep coming back.
- Will Rogers

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

root wrote on 7/24/2017 1:00 PM:
rickman wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?

--

Rick C

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)
 

On Mon, 24 Jul 2017 19:23:40 -0400, rickman wrote:

root wrote on 7/24/2017 1:00 PM:
rickman wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)
 

On Mon, 24 Jul 2017 22:32:54 -0400, rickman wrote:

wrote on 7/24/2017 9:46 PM:
On Mon, 24 Jul 2017 19:23:40 -0400, rickman wrote:

root wrote on 7/24/2017 1:00 PM:
rickman wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

What was the lowest speed you could use 5th gear in the 75 car?
Can't remember for sure, but it was a DOG at 60mph - requiresd a
downshift to get anywhere. I think hey geared the 75 GT the same as
the 4 speed. I know I was shocked by the mileage on that trip - going
out to Kingston to pit crew for Taisto Heinonnen, "The Flying Fynn"
and Tom Burgess on the Twin Lakes Rally. Crewsd for him on the Tall
Pines and the Blossom too.

I was offered his backup Celica Team car in 1980 when we finished
rallying in the navigational rallye series (After finishing 1st,
second and third in 3 years we were no longer elligible) and our R12
was not adequate to run competetively in the performance series but I
decided to quit while I was ahead, since I was getting married.

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On Monday, July 24, 2017 at 9:46:01 PM UTC-4, wrote:
On Mon, 24 Jul 2017 19:23:40 -0400, rickman wrote:

root wrote on 7/24/2017 1:00 PM:
rickman wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power.

Woosh! That would seem to be exactly the man's point, that it's
then a squaring of two, not cubing of three.

What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)
 

On 07/24/2017 07:46 PM, wrote:
An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

I had the misfortune to own a '71 Audi when the 55 mph national speed
limit went into effect. The German engineers thought 55 was a very brief
period on your way to cruising speed not a speed you'd try to drive.