MPG, km/L and L/100 km explained
Understand reciprocal fuel economy and the difference between US and Imperial gallons.
Economy versus consumption
MPG and km/L measure distance per fuel amount: higher is better. L/100 km measures fuel per distance: lower is better. The relationship is reciprocal, so doubling MPG halves fuel used for the same distance.
US and UK gallons are different
A US liquid gallon is 3.785411784 litres. An Imperial gallon is 4.54609 litres. A UK MPG figure is therefore larger than the US MPG figure for the same vehicle efficiency.
Formulas
L/100 km = 235.2145833333333 ÷ MPG US, or 282.4809363318222 ÷ MPG UK. km/L = 100 ÷ L/100 km.
Example calculation
7 L/100 km is approximately 14.29 km/L, 33.60 MPG US, or 40.35 MPG UK. Over 300 km it uses 21 litres at that constant consumption.
Use measured totals
For several trips, divide total fuel by total distance rather than averaging their MPG figures. Keep the measurement boundary consistent and account for differences in refill level.
Use the fuel economy converter for units, fuel economy calculator for measured fuel and distance, or fuel cost calculator for a budget. Definitions: NIST SI guidance.