Gas mileage calculator

Updated September 2026

How to use it

Gas mileage is measured between two full tanks. There is no way around that, because a fuel tank is the only measuring cup you have.

1. Fill the tank all the way. Write down the odometer.
2. Drive normally. A few hundred miles is ideal, a full tank is fine.
3. Fill up all the way again, at the same pump if you can, and note the odometer and the gallons.
4. Put both readings and the gallons into the calculator above.

The same pump matters more than people expect. Pumps cut off at slightly different points, and on a fifteen gallon fill a half-gallon difference in where the nozzle clicks is a three percent swing in your answer.

The formula

In plain arithmetic, with no units to trip over:

A worked example. You fill up at 42,150 miles. You drive for a week and fill up again at 42,462 miles, and it takes 11.6 gallons.

• Distance: 42,462 − 42,150 = 312 miles
• Fuel: 11.6 gallons
• 312 ÷ 11.6 = 26.9 MPG

At $3.29 a gallon that tank cost $38.16, which is 12.2 cents a mile. Drive 12,000 miles a year at that rate and fuel alone is about $1,470.

Units, and why yours might not be MPG

Three units are in common use, and they do not all point the same direction, which is where the confusion starts.

The imperial gallon trap catches people every week. A UK car advertised at 45 MPG is 37.5 MPG in American terms, because an imperial gallon is 4.55 litres against the US gallon's 3.79. That is not the car being better. It is the bucket being bigger. The calculator above handles all four, so pick your unit before you type.

Four ways people get the number wrong

Using the wrong fill-up's gallons

The gallons that go with a distance are the ones you put in at the end of it. Using the first fill-up's gallons measures a tank you burned before you started counting.

Topping off, or not topping off

Be consistent. Click off and stop, every time, or top off to the brim every time. Mixing the two is what produces a run of results that bounce between 24 and 33 for no visible reason.

Counting a partial fill

Twenty dollars' worth on a Tuesday is not a measurement. It is fine to record it for spending, but it cannot produce an MPG figure, and averaging it in drags the whole history sideways.

Judging the car on one tank

A single winter tank of short cold trips can read 20 percent below the same car's summer highway average. Both numbers are real. Neither is the car. Take the average across five or more tanks before you conclude anything.

MPG conversion table

The common figures, rounded, so you do not have to reach for the calculator for a rough answer.