Fuel Economy Converter (mpg ↔ L/100km)

Converting fuel economy

Fuel economy is measured two opposite ways, and that's what makes it genuinely confusing. Some countries quote distance per fuel— miles per gallon, kilometres per litre — where bigger is better. Others quote fuel per distance — litres per 100 km — wheresmaller is better. The two are reciprocals, so you can't convert by multiplying; you have to invert.

mpg (US) × L/100km = 235.215

mpg (imperial) × L/100km = 282.481

Those two constants are the whole trick: divide either one by the figure you have to get the other. The imperial constant is larger purely because an imperial gallon (4.546 L) is about 20% bigger than a US gallon (3.785 L), so the same car scores a higher "mpg" in the UK than in the US without using a drop less fuel.

Worked example

A car using 8 L/100km does 235.215 / 8 ≈ 29.4 mpg (US), or 282.481 / 8 ≈ 35.3 mpg (imperial) — same car, two different numbers. In km/L it's 100 / 8 = 12.5 km/L.

Why the reciprocal matters

Because the relationship is inverse, equal-sized mpg improvements are not equal fuel savings. Going from 15 to 20 mpg saves far more fuel over the same distance than going from 40 to 45 mpg, even though both are "+5 mpg". Fuel per distance (L/100km) is the more honest measure of what you'll actually spend, which is why most of the world uses it.

These measures run in opposite directions

Miles per gallon and litres per 100 km are reciprocals, not scaled versions of each other — in one, higher is better; in the other, lower is. They cannot be converted by multiplying, and because the relationship is a curve, equal-looking improvements are not equal: going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50. Note also that a US gallon is about 83% of an imperial one, so the same car has two different mpg figures depending on which side of the Atlantic quoted it.