Work out fuel economy converter instantly with clear inputs, formula shown and shareable results.
Fuel economy comes in two opposite flavours. Miles per gallon and km/L are 'distance per fuel' — higher is better. Litres per 100 km is 'fuel per distance' — lower is better. Because they are reciprocals, you cannot convert with a single multiplier: the calculator normalises the input to L/100 km first, then derives the rest. This also explains why saving fuel going from 15 to 20 mpg is a far bigger gain than going from 40 to 45 mpg, even though both are a 5 mpg step.
mpg (US) → L/100 km
L/100 km = 235.214583 ÷ mpg(US)
mpg (UK) → L/100 km
L/100 km = 282.480936 ÷ mpg(UK)
km/L → L/100 km
L/100 km = 100 ÷ (km/L)
The gallons differ. A US gallon is 3.785 L and an Imperial gallon is 4.546 L, so the same car scores about 20% more mpg on the UK scale than the US one.
Because fuel cost scales linearly with it. Halving L/100 km halves your fuel spend, whereas an extra 5 mpg means very different savings depending on where you started.