Fuel & Charging
True MPG Calculator
What your car actually returns, measured from one fill-up rather than estimated by the dashboard — and what the distance between that figure and the number on the window sticker costs you in a year.
Miles per gallon, measured
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— mi ÷ — gal
What the gap costs, per year
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— of the — MPG rating
- Miles driven on this tank
- — on — gal
- What this fill cost
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- Cost per mile, fuel only
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- Fuel per year, at this MPG
- — — gal
- Fuel per year, had it hit its rating
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- The difference, per year
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Fuel only. This is what the miles cost to move the car and nothing else — before depreciation, insurance, maintenance or tires, which together cost most drivers considerably more than gasoline does. Falling short of an EPA rating is normal and is usually about how and where the car was driven, not a fault: the rating comes from a standardized laboratory test, and the government that publishes it says plainly that it “may not accurately predict the average MPG you will get.”
Informational only, not professional advice. This tool divides miles by gallons using figures you supply. A single tank is a small sample and a low reading is not by itself evidence of a mechanical fault — if you suspect one, that is a question for a mechanic, not a calculator. Nothing you type is sent anywhere; the arithmetic runs in your browser.
How this is calculated
One division, and then the money that follows from it:
miles driven = odometer now − odometer at the last fill
MPG = miles driven ÷ gallons added at this fill
cost per mile = $ per gallon ÷ MPG
gallons a year = miles per year ÷ MPG
cost a year = miles per year × cost per mile
the gap = cost a year at your MPG
− cost a year at the rated MPGThe fill-to-fill method, and its one trap
This is the procedure the Department of Energy and EPA publish on fueleconomy.gov, word for word: fill the tank completely and write down the odometer; drive; fill it completely again and write down both the new odometer reading and the gallons it took. Subtract to get the distance, then “divide the number of miles driven by the number of gallons it took to fill the tank.” Their worked example — an odometer moving from 32,645.1 to 33,001.3 on a 13.5-gallon refill, giving 356.2 miles and 26.4 MPG — is pinned as a test against this site's own arithmetic, so if this page ever stops agreeing with its source, the build fails.
The trap is which fill you count. The gallons that matter are the ones added at the end of the measured distance, not the beginning. Refilling to the same point replaces precisely the fuel burned since the last time you refilled to that point, which is what makes the method work without ever knowing the tank's size or how full it was. Counting the first fill instead measures a distance against fuel that was mostly burned before the distance started.
Where the error lives
Not in the arithmetic — in the fill. A pump's automatic shutoff triggers at slightly different points depending on the nozzle, the flow rate, the temperature and how level the car is sitting, and the odometer itself carries a small error of its own. On a 13-gallon fill, a third of a gallon of inconsistency is over 2% of the answer. That is why the panel above calls a single reading a rough figure: it is measuring something real, but not to the tenth of an MPG that the display implies. Stopping at the first click every time, at the same pump, makes the error repeat rather than wander — and a repeated error largely cancels out when you compare one tank to the next.
What the EPA rating is, and what it is not
The number on the window sticker comes from standardized laboratory tests designed to represent typical driving. EPA states the limit of that plainly: the ratings “are a useful tool for comparing the fuel economies of different vehicles but may not accurately predict the average MPG you will get”. It is a like-for-like comparison figure, not a promise. Coming in under it is the normal case, and this tool treats the difference as information about how and where the car is driven rather than as a verdict on the car.
One detail worth knowing if you compare against the combined figure: it is not the average of city and highway that it looks like. Under 40 CFR 600.210-12, combined fuel economy is the harmonic mean of the city and highway values weighted 55/45 — 1 ÷ (0.55/city + 0.45/highway) — which always comes out lower than the arithmetic average several consumer sites publish. If the tank you measured was nearly all highway, compare it against the highway rating instead; against combined you are measuring your driving mix as much as your car.
Why a normal-looking shortfall is worth pricing
A gap of three or four MPG sounds like a rounding error and reads like one on a dashboard. Carried out over a year of driving at your own pump price, it stops sounding like one — which is the entire reason this page ends in dollars rather than in a percentage. Published figures give a sense of what moves it, all from the DOE and EPA: aggressive driving costs roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic; gas mileage “usually decreases rapidly at speeds above 50 mph,” with each 5 mph over 50 the equivalent of paying about $0.29 more per gallon; cold weather costs a conventional gasoline car about 15% in city driving at 20°F against 77°F, and up to 24% on short 3-to-4-mile trips; and E10 gasoline returns 3% to 4% fewer miles per gallon than ethanol-free fuel. Several of those stack.
What this does not cover
Fuel and nothing else. The cost per mile here is energy only — before depreciation, insurance, maintenance, registration and tires, which together cost most drivers considerably more than gasoline does; that whole picture is what the True Cost per Mile Calculator is for. The method also assumes both fills went to the same point in the same tank, so it cannot survive a partial fill, a fuel-up at a different-behaving pump mid-measurement or a long stretch of idling that burned fuel without turning the odometer. Plug-in hybrids will produce very high and perfectly genuine figures here, because the miles driven on electricity are counted while the electricity is not — for a plug-in, this measures the gasoline half of the story alone. And a single tank is a sample of one; nothing on this page is a diagnosis of a mechanical problem.
Last reviewed: August 2026
Frequently asked questions
Which gallons do I enter — the ones I just bought, or the ones from last time?
The ones you just bought, at the fill that ends the measurement. This is the single most common way the calculation comes out wrong, so it is worth being concrete. You fill the tank and write down the odometer. You drive. You come back and fill the tank again, and it takes 13.2 gallons. Those 13.2 gallons are the fuel you burned over the distance you just drove, because topping back up to the same point replaces exactly what left the tank. The gallons you bought at the first fill are not a measurement of anything — they only tell you how empty the tank was before, which the calculation never needs. Nothing about the tank's size, or how full it looked, enters into it.
My reading is well below the EPA number. Is something wrong with my car?
Probably not. Falling short of the label is normal, and the government that publishes the label says so: EPA's own guidance is that the ratings "are a useful tool for comparing the fuel economies of different vehicles but may not accurately predict the average MPG you will get." The estimates come from standardized tests meant to represent typical driving, and almost nobody drives typically. Speed alone accounts for a lot — fuel economy usually falls rapidly above 50 mph, and the DOE puts each 5 mph over 50 at roughly the same as paying an extra $0.29 a gallon. Aggressive driving costs 15% to 30% at highway speeds. Cold costs about 15% in city driving at 20°F versus 77°F, and as much as 24% on short 3-to-4-mile trips. E10 gasoline gives 3% to 4% fewer miles per gallon than ethanol-free fuel. A tank that combined a cold month, a short commute and a heavy right foot can land 25% below the sticker with a perfectly healthy engine. What is worth investigating is a sudden change: a car that returned 28 MPG all year and now returns 22 on the same route is telling you something the label never could.
Why not just use the average-MPG readout on the dashboard?
Use it, but check it with this. The trip computer estimates consumption from how long the fuel injectors are held open and how far the wheels have turned; it never sees a pump. This method sees nothing else — it is money you paid for a volume you received, divided by a distance you covered. When the two disagree, the pump is the one holding the receipt. Running the fill-to-fill calculation two or three times tells you your own car's offset, and after that the dashboard figure becomes useful again because you know which way to lean it.
How many tanks do I need before the number means anything?
Three or four, and then keep going. The weakness of this method is entirely in the fill: pump auto-shutoff triggers at slightly different points depending on the nozzle, the angle of the car, the temperature and how fast the fuel is flowing. Misjudging a 13-gallon fill by a third of a gallon moves the answer by more than 2%, which is larger than most of the differences people are trying to detect. Two habits shrink that error to almost nothing. Fill at the same pump, and stop at the first click every time rather than topping off — consistency matters far more than precision, because a repeated error cancels out of the comparison between tanks. And measure over longer distances: the same half-gallon of uncertainty spread over 600 miles is half the error it was over 300.
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