Miles per Gallon ↔ L/100 km

The two conventions are reciprocals with a conversion constant baked in, and the constant is exact: one hundred kilometres is 62.137 miles, and a US gallon is 3.785411784 litres, so $100 \ imes 3.785411784 / 1.609344 = 235.2146$. Multiply litres per hundred kilometres by miles per gallon and you always get that number.

Watch which gallon is meant. The imperial gallon used in the UK is 4.546 litres, about 20 % larger, so the same car rates 282.5 divided by its consumption in "imperial mpg" and looks more efficient than it is. This page uses the US gallon, the one on every American window sticker. Because the relationship is reciprocal rather than proportional, equal steps in one unit are not equal steps in the other, which is the whole point of the previous page's warning.