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

Actually it is - because you only make ONE calculation. The measurement accuracies do not change. The precision gets better, because you are not using numerous mileage measurements that can only be accurate to the closest 10th of a mile or KM (more accurate in the metric system becuase a KM is smaller than a mile) The precision is now 1/10km over 5000 km instead of 1/10 kn over 500, ten times.

Your accuracy on the fuel used is to the 10th of a liter (or gallon if you are a Yank)- again more accurate with the metric system because liters are smaller than gallons. So your accuracy is to the closest

10th of a liter 5 times - and the accuracy of the fillup is only the closest you can get it ONCE instead of 5 times.

Accuracy of fuel used will be, at the very most, 5X 1/10th liter more

- that's plus half a liter over to -0 liter under over 365 liters at

20 miles per US Gallon that's within better than 1.5% (1.369) at the outside.(assuming the calibration of the pumps is accurate - pumps are calibration tested on a more or less regular basis - when I was "in the business" we were still running imperial gallons for the most part

- the pumps were inspected and certified accurate to within 1/10 gallon in 5 gallons ( the closest the meter could read) IIRC when the switch to metric was made, it was 1/10 of a liter in 20 liters (the size of the test container remained virtually the same) so accuracy improved by roughly a factor of 4.

If I kept track of the fuel mileage on my vehicle over a period of

50,000 km, the accumulated average fuel economy could be easily calculated to within that percentage of error. ( I used to do that when I ran a vehicle log for business purposes)

Or consistently right. If you KNOW the accuracy of the speedo, it is a simple mathematical correction to achieve accuracy. The speedo may vary in accuracy because it is an inductive coupling device on a mechanical speedo, while the ODO will not vary as it is a direct geared connection to the driven wheels. With electronic speedos and odos, their calibration does NOT change. The only vatiance is tire wear ( aproxematelt 3/8 inch difference in diameter of a , say, 24 inch diameter tire, over it's lifetime - and that wear is pretty linear - so it is not rocket science to work in a correction for that too if you want to be a very anal engineer.

Or they could not - better to eliminate the randomization, or account for it in calculating accuracy.

I've averaged it over 240,000 miles - - -

I'm sure I could claim accuracy to closer than 1 MPG, but what good would it do over 240,000 miles??????? (and how could you prove me wrong?)

For COMPARISON testing, accuracy is not important - only precision and repeatability. On my electric conversion I could compare one type of tire to another by driving a given distance and route on one set, measure the watt hours of charge used, and compare to a different set of tires over the same route under the same conditions and KNOW how much better "mileage" I would get with one tire over the other.

Modifying the tune on my '63 Valiant, or a customer's Celica, or whatever - I could do a "before and after" run of 5 miles with my calibration can and know, to the ounce, how much more or less fuel was consumed over the same route, Using the "official" fuel mile tester I could measure to the cc over a half liter - that's an accuravy of 1 part in 500, or 0.2 percent. If I had a "rolling road" I could repeat the drive cycle accurately too - but I only had access to that at trade school (a chassis dynamometer) Not as easy to do today with fuel injected cars - but dynos are a LOT more common today than they were

40 to 45 years ago . . . and more programmable. If you know the cd of a vehicle today, a computer simulation can run a vehicle over a virtual course, correcting for ambient wind, changes in elevation, accelleration (knowing mass of the car) -every conceivable condition

- to make direct case to case comparisons EXREMELY accurate. (and fuel measurement technology has advanced so it's very simple to very accurately neasure the amount of fuel consumed as well - and also get very accurate measurements of instantaneous consumption - and with strain guages even know EXACTLY how much horsepower is being delivered to the road to figure out specific fuel consumption -

All stuff you "engineers" should understand.

Which can vary from no better than a SWAG to pretty darn close, even for the "mom and pop" or "hobyist" to EXTREMELY accurate for the engineer.

Whatever it is off, repeatability will be very close to 100%.

300 miles, repeatabilty to within less than 1/10 mile, or .2%

If he's as smart as the average fifth grader, within less than a cup at the same station, in the same spot, Lets nake it within a pint - that is 1/8 gallon in 15 - call it .8% on 15 gallons - - - so at LEAST within 1%

Again, repeatability within well under 1/10 gallon per fill (15 gallons) - about .6%?? Accuracy doesn't mean anything if the refill is at the same pump as the initial fill. Again, if you are in the "metric world" instead of the USA, accuracy improves by a factor of almost 4 on the fuel measurements, and 1.6 on the distance.

All of this accuracy can be accomplishes with NO special equipment.

Where the REAL fun comes in is duplicating the "drive cycle" - for the average driver, on the average road, "virtually impossible". If you have a closed course, and you are a VERY good and consistent driver, - perhaps you can get within a REASONABLE approxemation (repeatability over 5 runs, perhaps within less than 5%) - in open traffic, you'l be doing VERY good to get repeatability better than within 10% on a short run - over 300 miles, averaged over 5 runs (total 1500 miles) mabee within 1% on open road, or 5% running with traffic. Repeatability gets a LOT better over a short distance where fewer variables are involved. (Start from the gas station, accellerate to

30MPH in 2 blocks, enter the "expressway" and accellerate to 60mph by the first exit, maintain 60MPH to the 5th exit, slow down and exit the highway, stop. pull away from stop and re-enter the highway going the opposite direction, accellerate to 60 mph as you merge with traffic, decellerate from the 4th exit to the third exit, return to the gas station Repeat. Repeat Repeat.

That kind of driving can be very repeatable.

Fill with gas, Drive 3 blocks to the highway, drive 100 miles to "the city" Drive 5 miles across the city to restaurant, sports arena, or place of work, then return

Repeat Not much chance the 2 trips will be anywhere close. (I have had variances of more than 25% between 2 trips between Waterloo and Barrie Ontario over the same route, at the same time of day, and day of the week (and even the same month) - (even when the overall travel time was very close to the same for the round trip). generally WITHIN 10% IS PRETTY DOABLE - AND AT 20mpg THAT'S +/- 1 mpg at best

If you are in open country with very little traffic, repeatability gets MUCH better. Driving back and forth from Flagstaff to Jeddito Arizona you could likely repeat within well less than 1/4 MPG in the middle of the week

- or from Enid to Chickasha Oklahoma - or from Saskatoon to Regina.(very little traffic variability)

True of the speedometer, but NOT of the odometer. The odometer repeatabilty is as close to 100% as you will get even with a cable driven odometer. (it is a directly geared measuring device with ZERO vatiability - X number of cable turns per mile from the day it's made till the day it is scrapped ( generally 1000 turns per mile, but some older cars were 600 turns per mile, some motorcylses 1450, etc - but they never change) With electronic speedos and odos (virtually all cars today less than 15 years old) repeatability is almost 100%. Accuracy CAN be very close to 100% too, as on most cars under 10 years old today, the speedometer can be accurately reprogrammed to the tire diameter so repeatability is only affected by tire wear (mabee 3/8 inch in 24 over the life of the tire)

Directly related? but not necessarily 100% linearly related High manifold pressure (low vacuum) means heavy load which means poor mileage. The reverse is also true - but calibrating vacuum to MPG is virtually impossible with any level of accuracy. It WILL give you a good, better, worse indication though. Keep the vacuum up and you will get better mileage.

How many ingels can sit on the tip if a pin??? That's about how ridiculous this whole discussion is getting

Nearly all my driving is on secondary highways so I pretty much am driving at pretty optimal speeds for mileage although there are some traffic lights, they tend to be miles between stops. I have developed fuel efficient habits so I nearly always squeeze every last MPG on my trips. I have a manual, so I slip it out of gear and coast to lights and nearly always accelerate gently. I leave a lot of room to the car in front so I can ease up on the gas rather than hit the brakes. I think I am doing about as well as can be expected all the time, so my mileage seldom varies unless I do more city driving. High 19 or low 20 MPG, very consistent.

Mad Roger wrote on 7/21/2017 7:13 PM:

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. I had some larger tires at one point and it made the odometer read a bit low, also the speedometer.

BTW, someone said something about one being accurate meant the other was accurate and that is not necessarily true. My speedometer is mechanical and so has a separate calibration factor. With the present tires it reads a bit high, about 1 to 1.5 MPH at highway speeds. That one is harder to calibrate than the odometer (which is pretty much on point) because it is hard to maintain a constant speed for long enough to get an accurate reading even with the cruise control. But with lots of readings I am pretty confident these numbers are right.

So my odometer is accurate and precise.

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

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 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.

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

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

I think one time in nearly 20 years I got 22 MPG. I think I can count on my fingers the times I got 21 MPG. These days with nearly all my driving on the highway it is much less than 1 in 20 fills that I see less than 19 or even 19.5 MPG. It is nearly always just under or just over 20 MPG, more just under :-( If I were the dancing type I would have a little happy dance when it actually is over 20 MPG, lol. It makes my day.

I think the consistency of my MPG readings show how well each of these can be measured. As you say, the pump is going to be dead on. Other than scale error which can be calibrated out the odometer will be very good. Filling your tank can be good as well. It's not like they design gas tanks to have air pockets.

You don't need to know any of this specifically. You just need to measure your fuel mileage and measure the accuracy and precision of the results. Why do you care which of the three has what specific degrees of accuracy and precision? You care about the accuracy and precision in the result and you can measure that. Remember there are other factors as well that actually impact your MPG from tank to tank. They will show up when trying to measure any one influence so might as well calibrate them in too.

When you consider how the old speedometers worked it's amazing they came anywhere close to reality. I had a '60 Plymouth where the speedometer looked like a red bar progressing across a horizontal display rather than the usual needle. The guts were a tube about a foot long and an inch and a half in diameter suspended in bearings and loaded with a spiral spring. The mechanical cable from the tailshaft of the transmission tweaked the tube with each revolution via a magnetic link. It was an analog integrator with the spring controlling the tube's rotation.

The standard dial type was the same principal but the Chrysler engineers went out of their way to be weird. That was also the era of the pushbutton Torqueflite tranny and left handed lugnuts on one side.

A lot of modern speedometers are just as bizarre converting a perfectly good digital pulse train to an analog voltage to drive a dial rather than going straight digital.

But now

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.

snipped-for-privacy@snyder.on.ca posted for all of us...

+1

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.

I tried that one day on a flat stretch so there would be little variance. This was on my regular trip to work. Speed limit is 65. One day I did 70, the next 65, then at 60 is was dicey, the next day I tried

55 for about 30 seconds and decided not to risk my life.

I forget the details, but 60 was better than 70 by a couple of mpg. Problem is, I prefer driving 75. If I could get away with it I'd go 85+ but don't want to pay the fines.

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.

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

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

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.

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

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.

I'm sure you do believe that.

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

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

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

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

I'm sure you do.

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.

Define "very good" please.

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.

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.

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.

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.

...

I got curious myself on what the numbers revealed and looked at the NIST numbers again.

I computed an empirical cdf and compared it to normal...statistics from the 20,036 observations are below:

I then compared to normal on the same plot and as outlined above N(mean,std) is too long-tailed on both ends in comparison. It turns out that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.

Anyway, from the above it's simple enough to get some pretty good estimates of what pump volume errors one might expect...the table below is from the empirical cdf NIST data...

P error(in^3)/5Gal error(%)

0.001 -22 -1.82 0.005 -9 -0.78 0.010 -8 -0.69 0.025 -6 -0.52 0.050 -5 -0.43 0.250 -2 -0.17 0.500 0 0 0.750 2 0.17 0.900 4 0.34 0.950 5 0.43 0.975 6 0.52 0.990 7 0.60 0.995 10 0.86 0.999 22 1.82

From the above, one can conclude the pump metering error small for all except the extreme outlier pumps.

The man is right You are wrong. You ASS U ME too much - and at the risk of insulting the few GOOD engineers on the list, you OBVIOUISLY are an "engineer", but not one I'd hire for a job. The job would come in WAY over budget, WAY late, and would need to be completely redone by techitians and technologists at great cost, or to save time and money, completely decommissioned and scrapped - starting over with someone who knew what thet were doing, and how to do it - engineer or not.

a whole lot of crap snipped

Roger, me lad - you wouldn't happen to be a britiah trained engineer, now, would you?? In what discipline of engineering?

I love that you are the only one quoting actual numbers and not pulling them out of your butt to answer the question!

But your numbers confuse me because they seem to be in cubic inches. You also mentioned that metric pumps are more accurate, but that's impossible, simply because the pump is as accurate as the pump can get, which, we can assume, is a mechanical thing (and not a metric thing).

All you're saying is that a liter is four times smaller than a gallon so the error is four times less for a given liter versus a given gallon but that's not saying it's more accurate. It's just saying the volume is less so the resulting error is less.

Anyways, can you just summarize what the error is for a typical USA pump in gallons?

For a typical 20-gallon fill, how many gallons off can reality be, plus or minus from the indicated reading on the pumpmeter?

At under 70 my car usually is in the 35 mpg + range; at 80, it is more like 32. I get even better mileage in Oregon with its 55 mph speed limit. I also get bored out of my mind. There isn't a whole lot of anything between Ontario and Bend but I figure as soon as I get up to a decent speed a OSP cruiser will materialize from the sagebrush.

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