How to calculate your car's fuel economy
Fuel economy tells you how far your car goes on a given amount of fuel. Knowing your real number, not the one on the sticker, is the easiest way to spot a problem early and to see what driving really costs you.
Updated
The full-tank method
This is the most reliable way to measure fuel economy, because it does not depend on the car's dashboard estimate.
- Fill the tank until the pump clicks off and write down the odometer reading.
- Drive as you normally would until the tank is at least half empty.
- Fill the tank to full again at the same kind of pump, and write down how much fuel it took and the new odometer reading.
- Subtract the first odometer reading from the second to get the distance driven, then use one of the formulas below.
Always compare full tank to full tank. If you skip a fill-up record, the next calculation will be wrong, so log every refuel.
The formulas
km/L = distance (km) ÷ fuel (litres)
L/100 km = fuel (litres) ÷ distance (km) × 100
MPG (US) = distance (miles) ÷ fuel (US gallons)
For example, if you drove 520 km and the refill took 40 litres, you used 7.7 L/100 km. That is 13.0 km/L, or about 30.6 MPG in US gallons.
Converting between units
Countries use different units, and L/100 km runs in the opposite direction from MPG: a lower L/100 km number is better, while a higher MPG number is better.
- MPG (US) = 235.2 ÷ L/100 km
- MPG (UK) = 282.5 ÷ L/100 km
- km/L = 100 ÷ L/100 km
- US gallons and UK (imperial) gallons are different sizes, so MPG figures are not directly comparable across the two.
Why your number changes
- Short trips and cold starts use more fuel than steady highway driving.
- Winter temperatures, winter fuel blends, and running the heater or air conditioning.
- Tire pressure that is too low, a roof box, or extra weight in the car.
- Fast acceleration and high speeds.
Track the figure over several fill-ups rather than reading too much into one. A steady slide over a few months can point to under-inflated tires or a maintenance issue worth checking.