Mileage calculator
Two numbers from your last fill-up tell you what your car is really doing.
Fuel economy is distance divided by fuel used: kilometers divided by litres gives km/L, and miles divided by gallons gives MPG.
Fill the tank, reset the trip meter, drive normally, then fill up again. The litres or gallons on the second fill are your fuel used.
How to measure it properly
The number on your dashboard is a computer's estimate. The tank-to-tank method below is what road testers use, and it is the only figure that includes everything your car actually did.
- Fill the tank until the pump clicks off. Do not top it up further.
- Reset the trip meter.
- Drive normally until the tank is at least half empty — the longer the better, because a short sample exaggerates errors.
- Fill up again at the same pump if you can, again stopping at the first click.
- The litres or gallons on that second fill are your fuel used. The trip meter is your distance.
The formulas
MPG = miles driven ÷ gallons used
L/100km = 100 ÷ km per litre
Note that L/100km runs backwards compared with the other two: a lower number is better, because it is fuel consumed rather than distance travelled.
What a falling figure is telling you
Track this over several tanks and a downward trend is worth investigating. The usual culprits, cheapest first, are under-inflated tyres, a clogged air filter, a roof rack or box left on, and unnecessary weight in the boot. Cold weather and short journeys also hurt, so compare like with like before assuming something is wrong.
Need the same figure in different units? Use the MPG and km/L converter.
Next steps
Common questions
- How do I calculate my car's mileage?
- Divide the distance travelled by the fuel used. Kilometers divided by litres gives km/L; miles divided by gallons gives MPG.
- Why does my dashboard show a different figure?
- The dashboard number is calculated by the car's computer from injector timing, and it tends to read optimistically. A tank-to-tank measurement includes everything the car actually did, including fuel used while idling.
- How many tanks should I measure to get a reliable figure?
- At least one full tank, and preferably three consecutive ones averaged together. A short sample exaggerates the error caused by how precisely each fill-up stopped.
- What does a falling fuel economy figure mean?
- Usually something simple: under-inflated tyres, a clogged air filter, a roof rack left on, or extra weight being carried. Cold weather and short journeys also reduce economy, so compare similar conditions before assuming a fault.