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

[Mad Roger]

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.

[dpb]

...

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.

[dpb]

On 07/22/2017 6:42 PM, Mad Roger wrote: ...

...

NO! That is not at all what any of those people said. You're simply repeating the same contention which is ok as far as it goes in that a point estimate is not the same as averaging or using other techniques to increase the precision of the measurement.

They (and I in particular) only point out that averaging muddles out the differences in intermediate filling levels.

The alternative measurement increases the denominator in the computation at the expense of no additional error in the measurement of the quantity used; the errors in the intermediary quantities in levels cancel identically because all the fuel gets used and the final level discrepancy is only the one but is is now related to the total quantity instead of the single.

There has been nobody I've seen who's claimed a bias error will be anything but that -- but in this case that one is a simple calibration and correction that will not add appreciable error if made. And even if not won't have any bearing on differences in performance over various driving conditions for the same vehicle in terms of seeing changes in that vehicle's relative performance. That it is off by whatever that percentage in mileage is from the actual is too obvious to belabor and nobody here has made any claim to the contrary.

[clare]

and exactly how is that germaine to the issue at hand? The speedo means NOTHING. All we care about is the ODO - which will ALWAYS be consistent, even in inaccuracy - so can be easily compensayed

[clare]

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.

[clare]

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?

[trader_4]

Oh no..... Just yesterday Clare had found a new BFF, they were ready to get a room together. Now they're bickering big time.

ROFL.

[trader_4]

:)

ROFL

[trader_4]

+1

I'm not sure I;d even call it averaging, it's a different test method. Two ways of doing it:

1 - Fill the tank once, drive until it's near empty, fill it again. Your accuracy is greatly affected by your ability or inability to fill it to exactly the same level. If you're off by a gallon on a 15 gallon tank, it;s 7%.

2 - Fill it at the beginning, drive it a much longer distance, through 10 tanks worth of gas where you have the pump reading on all of those, then fill it the last time to as close to the original fill as possible.

Method 2 reduces the inaccuracy due to not filling it to exactly the same level by an order of magnitude. If you're off by a gallon between the first and last fill, it's an error of ~0.7%. You still have whatever the accuracy of the pumps are to deal with, but I agree I'd trust that the pumps are going to be a lot closer to the 0.7% accuracy than 7%. I'd think they are better than 0.7%. And like you said, in between the first and last fill, it doesn't matter if you fill it all the way or only half full, etc.

[Mad Roger]

While the odometer has a decimal place, the tripmeter generally does not.

So anyone using the tripmeter/pumpmeter calculation has no scientific right to include the decimal place in the mpg calculation.

It's (mathematically) impossible to calculate 19.5 mpg when the tripmeter reading doesn't have a decimal place.

Likewise, it's (mathematically) impossible to calculate 19.5 mpg when the fill-level estimation isn't accurately known to some vague concept of less than a gallon.

The only reading, if the three required, that is reasonably accurate (someone quoted some figures already for the pumps, which I appreciate), is the pumpmeter itself.

But that pumpmeter reading is completely dependent on the fill-level estimation, which isn't known to any reasonable degree of accuracy.

I agree the way to get around the horrible fill-level inaccuracy is to average over numerous tankfulls, which helps greatly, but doesn't eliminate the two major inaccuraciesa which make it impossible for a mom-and-pop tripmeter/pumpmeter calculation to have a decimal place in the result.

[Mad Roger]

OK. Maybe I got it wrong.

Still, some people claim 19.5 miles per gallon, which I posit is an impossible level of precision given the tripmeter/pumpmeter method, even when taken over 10 tank fills.

Most tripmeters don't even have a decimal place, so, you can't include decimal points in the calculation.

Worse, the fill-level estimation is crude at best, where again, there is no decimal point.

It's a mathematical fact that if your measurements don't have decimal points in them, then your answer can't have them either.

Anyone quoting MPG with a decimal point has to first get those decimal points into the measurements!

[Mad Roger]

I agree that averaging the "fill level" estimate is a great way to reduce that huge error of guessing where the last fill level was.

I never disagreed with that, although I may not have realized it in the very beginning. So, we have to assume a 10-tank fill when we state what our innacuracies are.

I'm ok with assuming a 10-tank fill.

But remember, your inaccuracy is no better than your worst measurement, so, what do we do about a tripmeter that has no decimal point?

I will wager that most people use the tripmeter and not the odometer.

If the tripmeter/pumpmeter has no decimal point in the numerator, you can't possibly get a decimal point in the resulting calculation.

That's how math works. Isn't it?

So it's not "me" you'd argue against. It's math you have to argue against, since I'm just the messenger.

[Mad Roger]

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?

[trader_4]

Math challenged too, I see. I have 3 apples. I take one away. What percentage is left? According to the above, I can't determine that what's left is 66.66%

You certainly can include decimal points, the accuracy is affected by the total distance traveled. If you travel 10 miles, then yes the fact that you don't have tenths has a big effect. If you drive 100 miles it matters by an order of magnitude less. If you travel 10,000 miles, three orders of magnitude less and you have several decimals of precision.

No, worse is that it's been explained to you by what?, 3 people now, that by filling the tank many times on a longer trip, the fill level issue only matters on the last fill and the inaccuracy due to that is greatly reduced by all the other fills being measured by the gas pump, which is highly accurate.

See the above examples.

Just stop already.

[trader_4]

You told us the other day you were some kind of scientist, yet cubic inches confuse you?

"NIST tolerance is 6 in^3 in a 5 gal measure"

231 cubic inches in a gallon. 6/(231*5) = .005 or 0.5%

You really should just stop already.

You told us the other day you're some kind of scientist. DPB told you the NIST

[rbowman]

I cannot address 'most' tripmeters only those on the five vehicles I own. They all read to tenths of a mile.

I don't usually pay that much attention but with this thread in my mind when I fueled the bike today I noticed the gallons on the pump had not one but three decimal places. I cannot attest to the accuracy. I don't know what criteria the inspector uses when he places his seal on the pump.

Like I said, I have all sorts of decimal places available.

[rbowman]

I'll go out on a limb here -- where do we find a trip meter with no decimal place? In fact, before trip meters became ubiquitous, the odometer showed tenths.

This is similar to the panel in my '86 F150. Early in the game both the odometer and trip meter were mechanical and read tenths.

Think about it. In a newer car where the odometer only shows miles, how do you follow directions like 'turn right four tenths of a mile past mile marker 37' ?

[rbowman]

Please post specific car models where that is the case.

[rbowman]

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.

[clare]

Backwards, my boy. Every car I have ever owned with a trip odo has tenths on the trip - many (or at least a few) with trips do not have tenths on the main ODO

Get a LIFE!!

But it DOES - and you don't have to use the trip odo if you don't want to. Use a pen and paper (you remeber those?) and write down the start mileage when you do the first fill up. Keep track of all fuel added AFTER that mileage, and at the last fillup, total all the fuel added, write down the current mileage, and subtract (as an engineer you DO know how to subtract, right??( - the difference is the total mileage covered. Devide that number, complete with tenths,by your total fuel consumed, to the tenth of a unit, and you have your accurate fuel consumption, to the tenth of a MPG ( or in the case of a car calibrated in Km, to the tenth of a Km/liter - which is a WHOLE LOT more accurate.

But if you are as smart as the average fifrg grader you KNOW it is possible to get a LOT closer than that. Fill till the fuel is visible in the filler neck, at the lead free gas restrictor plate, and you are accurate to within about half a cup full in the vast majority of cases.

Bull crap