MPG Calculator

How to calculate MPG from a single fill-up

Miles per gallon is one division: miles driven divided by gallons burned. The hard part is not the arithmetic, it is getting two honest numbers. The reliable way is the fill-to-fill method, and it takes two visits to the pump. Fill the tank until the nozzle clicks off, reset the trip odometer to zero, then drive the way you normally drive. At the next fill-up, use the same pump if you can, fill to the first click again, and write down two things: the trip meter reading and the gallons on the receipt.

Those gallons are exactly the fuel you burned over those miles, because you started full and finished full. That is why the method works and why a half-tank estimate does not.

Worked example: a commuter sedan

You reset the trip meter at a full tank, drive for a week and a half, and the pump takes 12.5 gallons to fill you back up. The trip meter reads 320 miles. Your economy is 320 divided by 12.5, which is 25.6 MPG. In UK terms that is 25.6 x 1.201 = 30.7 imperial MPG, and in metric it is 235.215 divided by 25.6 = 9.19 L/100 km. Three ways of stating the same car.

Turning MPG into money

At 3.60 per gallon, cost per mile is the price divided by the economy: 3.60 / 25.6 = 0.141 per mile, or roughly 14 cents. That tank cost 12.5 x 3.60 = 45.00. Now scale it to your life. A 40-mile round-trip commute for 22 working days is 880 miles a month, which at 14 cents a mile is about 124 a month, near 1,490 a year in fuel alone.

Run the same numbers for a car that returns 34 MPG and cost per mile falls to 0.106. The same commute costs about 93 a month. That 31-a-month gap is what people are actually asking about when they search whether a more efficient car is worth it. Notice also that the saving from 15 to 20 MPG is bigger than the saving from 30 to 40 MPG over the same distance, because MPG is a ratio and its benefit flattens out. This is the well-known MPG illusion, and it is the reason regulators quote L/100 km or gallons per 100 miles, where the scale stays linear.

US gallons, imperial gallons and why the numbers differ

A US gallon is 3.785 litres. A UK imperial gallon is 4.546 litres, about 20 percent larger. A car sold on both sides of the Atlantic therefore carries two different MPG figures for identical physical performance: 40 MPG in the US is 48 MPG in the UK. If you are comparing a review from one country with a spec sheet from the other, convert first or the comparison is meaningless. This calculator shows both from the same input so you never have to do that step manually.

Quick reference

Why your number will not match the window sticker

EPA and WLTP figures come from standardised test cycles run on a dynamometer at controlled temperature with a trained driver. Your commute has hills, headwinds, cold starts, roof bars, a boot full of gear, traffic lights and a right foot with opinions. Real-world results commonly land 10 to 20 percent below the combined sticker figure, and short winter trips can be far worse because the engine spends most of its time warming up.

Other sources of noise worth knowing about. Trip computers are usually optimistic by 2 to 5 percent, so trust the pump over the dashboard. Auto shut-off points vary between pumps, which adds a fraction of a gallon of error to any single measurement. Winter blend fuel and ethanol content carry slightly less energy per gallon. And a tank measured mostly on the highway will flatter you compared with the same car in town.

None of this makes the calculation useless; it just means one tank is a data point, not a verdict. Log three or four consecutive fills and average them. That average is the number to use for budgeting, for comparing cars, and for checking whether a maintenance issue has quietly started costing you money. A sudden drop of more than 10 percent with no change in driving pattern is worth investigating: underinflated tyres, a dragging brake, a failing oxygen sensor or a thermostat stuck open are the usual suspects.