Gas mileage calculator

How do I calculate my gas mileage?

Fill the tank and reset the trip meter. At the next fill-up, divide the miles driven by the gallons it takes to fill up again: 300 miles on 10 gallons is 30 mpg.

How do I convert mpg to liters per 100 km?

Divide 235.215 by the mpg: 30 mpg = 235.215 ÷ 30 = 7.84 L/100 km. The same division works the other way. Lower L/100 km means better economy.

Why is UK mpg higher than US mpg?

The UK (imperial) gallon is 4.546 liters, about 20% more than the US gallon, so the same car gets about 1.2 × the mpg in UK figures: 30 US mpg is 36 UK mpg.

What does my fuel cost per mile?

Divide the price per gallon by the mpg: at $3.50 a gallon and 30 mpg, each mile costs 11.7¢, and 1,000 miles cost $116.67.

Why is my mileage lower than the EPA sticker?

Short trips, cold weather, highway speeds over 60 mph, air conditioning and roof racks all cut mileage. The EPA estimates on fueleconomy.gov come from standard lab cycles.

Why should I average several fill-ups instead of one?

A single tank is affected by where the pump shuts off, the temperature and how you drove, which can shift the result by a few mpg. Add the miles and the gallons from three or more fill-ups and divide the totals: 1,236 miles on 43.8 gallons is 28.2 mpg, a steadier figure than any one tank.

Can I use the car’s trip computer instead?

Use it as a rough guide. A trip computer estimates fuel use from the engine’s data rather than from what you pay for, so compare it against a full-tank calculation once or twice to see how far off it is. The fill-up method measures the fuel you actually bought.

The formula

Miles per gallon = miles driven ÷ gallons added at the next fill-up, provided the tank is filled to the same point both times. Outside the US, economy is usually given as liters per 100 km, where lower is better; the conversion is 235.215 ÷ mpg, which comes from 100 km ÷ 1.609344 km per mile and 3.785411784 liters per US gallon.

Worked example: one tank

You fill the tank, reset the trip meter and drive 412 miles. The next fill-up takes 14.6 gallons. Mileage is 412 ÷ 14.6 = 28.2 mpg. In metric units that is 663 km on 55.3 liters, or 8.3 L/100 km (235.215 ÷ 28.2).

Common mistakes

The biggest error is a partial fill. If the tank is not filled to the same level both times, the gallons added do not match the miles driven, so stop at the pump’s automatic shut-off at both fill-ups. A different pump can shut off at a different level, so use the same one when you can. Do not compare a US-gallon figure with a UK one: the imperial gallon is about 20% larger, so the UK number is higher for the same consumption. Season, tire pressure and cargo also move mpg, so compare tanks driven under similar conditions.

Sources: the mile (1.609344 km) and US gallon (3.785411784 L) are exact definitions in NIST SP 811; fueleconomy.gov (U.S. Department of Energy and EPA) explains how label values are measured and why real-world mileage differs. Related: fuel cost calculator, MPGe calculator, mileage reimbursement calculator and EV vs gas calculator.