# 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 ``` km/L = kilometers driven ÷ litres used 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 - [convert that figure to MPG or L/100km](https://www.kilometer-mileage.com/fuel-economy-converter.html) - [price a journey using your economy figure](https://www.kilometer-mileage.com/fuel-cost-calculator.html) - [convert your fuel volume between litres and gallons](https://www.kilometer-mileage.com/litres-to-gallons.html) ## Common questions