How to calculate fuel consumption
The most reliable way to measure fuel consumption is the full tank method. It uses the fuel you actually put into the car, so it is not affected by an optimistic trip computer.
- Fill the tank until the pump clicks off and write down the odometer reading.
- Drive as you normally would. A longer interval (a full tank or more) gives a more accurate result.
- Fill the tank to full again and note the liters you added — that is the fuel you used.
- Subtract the first odometer reading from the second to get the distance driven.
- Enter both numbers above.
To get kilometers per liter, divide the distance by the fuel used. For miles per gallon, divide 235.21 (US) or 282.48 (UK) by your L/100 km figure.
Worked example
You drove 520 km between two full tanks and the second fill-up took 33.3 liters.
That is the same as 15.6 km/L, 36.7 mpg (US), or 44.1 mpg (UK). At 1.75 per liter, the fuel cost is about 11.21 per 100 km.
What about partial fill-ups?
If you add fuel without filling the tank between two full tanks, add those liters to the total fuel used for the interval. Only the full tank at the start and end matters for the distance. If you forgot to log a fill-up, skip that interval — it would make your consumption look better than it really is.
Doing this by hand every time is tedious. The Fuellow app logs full and partial fill-ups, calculates consumption between full tanks automatically, and excludes intervals with a missed fill-up.
Frequently asked questions
How do I calculate fuel consumption?
Fill the tank completely and note the odometer. Drive normally, then fill up completely again and note the liters you added and the new odometer reading. Divide the liters by the kilometers driven and multiply by 100 to get L/100 km.
What is a good fuel consumption?
It depends on the vehicle. Many modern small petrol cars use about 5–7 L/100 km, mid-size cars 6–9 L/100 km, SUVs 8–12 L/100 km, and many motorcycles 3–5 L/100 km. Compare your figure with your own history and the manufacturer rating rather than with other cars.
Is a lower or higher L/100 km better?
Lower is better. L/100 km measures how much fuel the vehicle needs to cover 100 km, so 5 L/100 km is more efficient than 8 L/100 km. With km/L and mpg it is the opposite: higher numbers are better.
Why is my calculated consumption different from the trip computer?
Trip computers estimate consumption from injector data and are often a few percent optimistic. The full-tank method uses the fuel you actually paid for, so it is usually the more reliable long-term figure.
