Fuel efficiency measures how far a vehicle travels per unit of fuel consumed. The US and UK both use "miles per gallon" but with different gallons: 1 US gallon = 3.785 L, 1 UK gallon = 4.546 L. So a car rated at 40 mpg US achieves only ≈ 33.3 mpg UK — the same vehicle with very different numbers depending on which standard is applied. Europe universally uses litres per 100 km (L/100km), which is an inverse measure: lower is better, the opposite of mpg.
The inverse relationship of L/100km creates a non-linear conversion with mpg. Improving fuel efficiency from 10 to 20 mpg saves the same amount of fuel per kilometre as improving from 50 mpg to infinity — the first step gives far more real-world benefit. This "mpg illusion" is why European regulators prefer L/100km, where equal numeric improvements represent equal fuel savings. This converter requires positive values; zero or negative inputs are not valid fuel efficiency figures.
| 30 mpg (US) | = 7.84 L/100km |
| 40 mpg (US) | = 5.88 L/100km |
| 50 mpg (US) | = 4.70 L/100km |
| 30 mpg (UK) | = 9.41 L/100km |
| 10 km/L | = 23.5 mpg (US) |
| 6 L/100km | = 39.2 mpg (US) |