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

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What is a realistic accuracy & precision of typical MPG measurements when measured by the consumer using the typical method of dividing their tripmeter miles by the gas-pump gallons during fillup?

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Close to 100% accuracy if done right. I've done it on long trips. But MPG will vary depending on terrain, weather, wind direction, stop-and-go traffic, etc. So if you want "true" MPG for your car, you have to do it for the life of the car. Once you do it initially, it's kind of pointless to do again except to satisfy your curiosity.

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Repeatabilty is terrible. Accuracy can be pretty good over multiple tanks. Can be pretty good even on single tanks IF there is a way to ensure the tank is always filled to EXACTLY the same point (like a level in the fill - tube, with the vehicle parked at EXACTLY the same place for each fill-up). Relying on the auto-shutoff of the pump can cause variance of several liters per fillup.

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My year old car gives constant mileage and appears very accurate when I calculate based on fill-up. It even has a moving mileage meter going up to 80 mpg when just coasting down hill or maybe 10 mpg going up hill. I recall many times on long trips mileage varying all over the map probably because I was filling up at different stations.

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IF the odometer is accurate and you do the math out to the 10ths of a gallon the pump shows it can be VERY accurate.

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Steve W.```
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Vehicle position or the auto shut off point won't make any difference. You read the amount of fuel you pumped off the pump itself. The only real issue is odometer accuracy. That can vary with tire size variations and factory calibration.

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Steve W.```
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That's only assuming you started with a literally empty tank and ran it empty.

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There is a difference between Accuracy and Precision.

Keeping it simple and consistent: Run the filler to the first click. Do NOT round to the nearest whole number. Note the miles.

Run the filler to the first click. Do the math.

You will be within a percent or two, certainly far more accurate than your driving is consistent. Meaning, that if you do this over 50 gallons of fuel , or so (about 2,410 miles in my case), you will have a single-decimal aver age MPG that is good enough for most uses.

My on-board computer calculates electric miles, regenerative miles and gaso line use with each trip at two decimal places. Doing check-math on paper ju st for giggles, my one-decimal calculation was dead-on via the method descr ibed - based on total miles driven and total gasoline used.

Peter Wieck Melrose Park, PA

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If you just do it one time, you can not be sure you put in the same ammount of fuel that was taken out.

If you keep a running total of the ammount of miles and fuel over several tank fulls , the ammount of fuel will sort of average out.

Say you park so the back of the car is up hill and you fill the tank. Go a number of miles and fill up again. This time the back of the car is down hill. You may burn out 15 gallons, but only put back in 14 gallons. Ot it could be the other way around and you burn 14 gallons, but only put back in 13 gallons. From tank to tank full there could be a large variation. Over many tanks, the variation will average out to a lessor error. After say 10 tanks used you only have to contend with one or two errors caused by the exact ammount of fuel put in the tank. Probably just the last tank full would be where the error would come in. So instead of 1 gallon of error like the example above, you would have about .1 gallon of error if the pumps are correct, which they should be.

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If you're getting MPG on a long trip, you only fill the tank all the way when you start and when you finish the trip. All the other gas stops you go by the meter reading. Very little room for error if you write down the meter reading.

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Trip meter miles depends on circumference of driving wheels. I know my speedo closely matches readings of roadside radar displays or my GPS, so I guess trip meter miles will be accurate too.

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fly shit to the left, pepper to the right.

Guys and gals - *EVERYTHING* depends on the accuracy of the odometer, the v ariation in the operating diameter of the tires, the amount of friction inv olved (LRR tires vs. AT tires as one example), even tire pressure. Then, ad d head or tail winds, external temperature, type and age of road surface, n umber of people in the car, number of dead McDonald's wrappers in the back, and so on and so forth.

If one wants five-decimal precision (that is also accurate) then one will n eed more tools than a commercial gas-pump reading and a simple odometer rea ding. However, if one wishes simply one-decimal accuracy, the problem is tr ivial and needs very little analysis-in-depth.

And, whether the car is parked on a hill pointing up, or pointing down, if it is being fueled at a legal, code-compliant fuel station, the flatness at the pump islands is regulated. Despite the many reaches to the contrary, t his ain't nohow rocket science! It barely rises to slide-rule requirements. .. .

Peter Wieck Melrose Park, PA

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No, steve. You are wrong. The amount of fuel you put in is the amount you can squeeze into the empty portion of the tank. The amount you used is the amount that used to be in the tank. You need to fill it to the exact same point each time to get an accurate reading. You may have filled your 72 liter tank to only 71 liters the last time you put in 50 liters to fill the tank. Now, at a different station, with different levels, you may squeeze in 73, or only 68. COSISTANCY is the key - and where most will fall down, because, like you, they just don't REALLY understand.

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If the first and last are identical, none of the others matter. The difference of 1, or 10 liters spread across many tanks becomes , more or less, just noise. On the short term, like 1 tank, it can be a pretty large percentage of error.

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You buy gas by the gallon and mileage is miles per gallon. Summer gas has higher density so you get more gas by weight for your money but it costs more per gallon in the summer. Sounds like you can't win.

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Every reading a mom and pop does has inaccuracies that, I posit, are tremendously higher than most people seem to think they are (at least most people who quote mpg figures with decimal places in them).

Most people have a tripmeter reading and a gas pumpmeter reading. Where they fill the tank and reset the tripmeter before driving away.

I can't find any reliable source that says what the accuracy or repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but basic logic dictates that the errors compound such that there is likely (IMHO) no way to get anywhere near decimal-point accuracy, and worse, probably plus or minus 1 mpg is the closest anyone can get in terms of repeatability and precision.

Even the EPA's \$360,000 machine only claims plus or minus 2% of the indicared reading. I can't find where I got the notion that a mom and pop can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg calculation - but I'm still seeking those numbers as we speak.

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On 07/21/2017 11:51 AM, Mad Roger wrote: ...

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Why do errors compound in your view?

And, it depends on what you mean in terms of accuracy -- in terms of absolute one needs to know the calibration error of the odometer; most folks are satisfied to just assume it's close enough for the purpose.

If you look at simply a single fillup, it's not unreasonable to expect a few tenths of a gallon difference between the first fillup level and the subsequent; if you try it on shorter distances than a full tank then the fractional error goes up.

OTOH, if one keeps track over longer periods of multiple fillups and take some care to use the same filling pattern and only fills up after using near the full tank capacity, then over time plus/minus targets _will_ tend to cancel out and I have no qualms in believing a relative performance number in the 0.1 mpg can be determined.

As noted, I've done this on long trips a number of times (generally on first trip or so with a new vehicle, either actually new (rare) or (most often) new to me) just to see how it compared with previous and have had quite good comparisons on recent ones with the computer-computed results. These would be over total distances of 1500 to 2000 miles, not just 20 miles test runs.

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+1

I measure my gas mileage on every fillup. I get 19 to 20 MPG every fill unless I do a lot of around town driving. Very consistent. I watch it to see if it drops off which would mean something is wrong.

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Rick C```
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The EPA doesn't have the time to do accurate MPG numbers Plus/Minus 2% is good enough for the EPA. I'm sure my "mom and pop" MPG number are more accurate than that. But so what? The MPG I get depend on the driving circumstances. For instance, I've measured my MPG on a number of cars on 3000 mile round trips to Florida. I can tell you the EXACT total MPG I got on those 3000 mile trips because I carefully noted the exact metered amount of gas I used, and I verified the odometer accuracy using mile markers. The only real useful thing that gives me is my MPG for the entire trip. That includes local traffic when getting off the highway, and my travels at my destination. But I know my approximate MPG at steady highway speed because I sometimes do tank to tank calculations by filling to the filler tube. That too is an EXACT calculation, but is still only approximate MPG because maybe the terrain and weather may vary. So before you ask about "accurate MPG" you have to define what that is.

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It's like a chain is no stronger than the weakest link.

No calculated result can be better than the worse inaccuracy.

Accuracy, precision, and sigfigs are standard terms:

Accuracy: how closely a measured value agrees with the correct value. Precision: how closely individual measurements agree with each other. Sigfigs: accuracy is no better than the least accurate measurement.

By way of off-the-cuff example, if the accuracy of the odometer is to the billionth of a mile and the accuracy of the pump gallons is to the billionth of a gallon, but the accuracy of the fillup is plus or minus one gallon, then the resulting mathematical (division or multiplication) accuracy can be no better than plus or minus one gallon.

A single fillup will never suffice.

We're trying to compare a MPG *change* between two situations, so, by definition, there _must_ be (at the very least!) /two/ separate calculations.

• Calculation before the change (say, smaller tire/wheel diameter)
• Calculation after the change (say, larger time/wheel diameter)

That's not necessarily true, because it depends on the understimations balancing out the overestimations, but I'm not going to quibble that more calculations done over time are likely going to randomize the precision and accuracy fluctuations over time.

While I will not quibble with your statement (because I essentially agree with you), I can point out that your speedometer can be consistently wrong in the same direction in either precision or accuracy, in which case it's

*not* going to balance out over time. It will be consistently wrong, over time.

But, let's not quibble about that because we both can assume that, for our purposes, the randomization of measurement results will be half the time underestimating and the other half the time overestimating - such that they could balance out.

Nobody yet, and even not me, has supported a claim for any better accuracy than my presumed plus or minus one mile per gallon using the standard mom-and-pop test of dividing the number of miles driven based on the tripmeter reading by the pump indication of gallons used to fill back up to a presumed similar previous starting point of amount of fuel consumed.

Remember, the resulting accuracy can't possibly be better than the least accurate measurement.

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