The number on a car’s window sticker is a lab result; the number that matters for your budget is the one your own car achieves on your own roads. The easiest way to find it is the full-to-full method: fill the tank, drive, fill it again, and divide the miles by the fuel you put back. This MPG calculator does that division, converts the answer to litres per 100 kilometres and kilometres per litre, and — if you enter the pump price — shows the fuel cost per mile.
How to use the gas mileage calculator
- Fill the tank until the pump clicks off, and reset the trip meter (or write down the odometer).
- Drive normally until the tank is at least half empty. The longer the stretch, the smaller the rounding error from the pump.
- Refill to the same point — ideally at the same pump and with the car parked the same way — and note the gallons or litres added.
- Enter the distance and the fuel. Use the unit menus if your trip meter is in kilometres or your pump is in litres.
Once you know your mpg, the fuel cost calculator turns it into the price of any trip, and the fuel economy converter switches between mpg, UK mpg, L/100 km and km/L.
MPG formula
L/100 km = litres used ÷ km driven × 100
km/L = km driven ÷ litres used
Fuel cost per mile = price per gallon ÷ MPG
The calculator converts every input to miles, kilometres, gallons and litres with the exact definitions (1 mile = 1.609344 km, 1 US gallon = 3.785411784 L), so you can mix units freely: kilometres driven and gallons bought give the same answer as miles and litres.
Worked example
You fill your tank, reset the trip meter and drive 310 miles. At the next fill-up the pump delivers 11.5 gallons at $3.50 per gallon.
- MPG = 310 ÷ 11.5 = 26.96 mpg.
- L/100 km = 235.215 ÷ 26.96 = 8.73 L/100 km.
- Fuel cost per mile = 3.50 ÷ 26.96 = $0.130.
- Fuel for the 310 miles cost 11.5 × $3.50 = $40.25.
Example fill-ups
These sample fill-ups show how the same arithmetic plays out for different driving. All values are calculated, not looked up.
| Miles driven | Gallons added | MPG | L/100 km |
|---|---|---|---|
| 250 | 10 | 25.0 | 9.41 |
| 310 | 11.5 | 27.0 | 8.73 |
| 380 | 12.2 | 31.1 | 7.55 |
| 420 | 14 | 30.0 | 7.84 |
| 450 | 18 | 25.0 | 9.41 |
Tips for an accurate reading
Pump click-off varies by nozzle, car angle and flow rate, so a single tank can be off by a few percent. Averaging three or more tanks smooths this out. Record your results in the same conditions — mostly city or mostly highway — if you want to compare tire brands, a roof rack or a new driving habit. Do not top off after the nozzle clicks; the extra can spill into the vapour recovery system and distorts the number.
If your mpg is well below the EPA rating for your model, check the basics: tire pressure (under-inflated tires raise rolling resistance), a dirty air filter on older cars, a stuck brake, extra cargo, and idling. The EPA and Department of Energy at fueleconomy.gov list the factors that affect gas mileage and how much each can cost you, and let you look up the combined, city and highway ratings for any vehicle. Mpg in cold weather and on short trips can be 15–25% below warm-weather highway figures, so a lower winter number is normal.
Sources
Unit definitions: NIST Special Publication 811. Ratings and fuel-economy factors: US EPA and Department of Energy, fueleconomy.gov.
Frequently asked questions
How do I calculate my car’s mpg?
Fill the tank completely, note the odometer or reset the trip meter, drive, then fill up again to the same point. Divide the miles driven by the gallons you added at the second fill-up.
Is the car’s onboard mpg display accurate?
It is usually within a few percent but often reads slightly optimistic. Checking against a full-to-full hand calculation over a few tanks is the best way to calibrate it.
What is a good mpg figure?
Most new gasoline cars in the US are rated about 25–35 mpg combined; hybrids reach 40–55 mpg. Large trucks and SUVs often rate 15–22 mpg. Compare your result with the EPA rating for your exact model at fueleconomy.gov.
Why does my mpg change from tank to tank?
Speed, temperature, short trips, idling, cargo, tire pressure and the amount of city versus highway driving all shift the result. Average several tanks to see your normal figure.
How do I convert my result to L/100 km?
Divide 235.215 by the mpg. The calculator shows L/100 km and km/L automatically.