Fuel Economy Converter

Convert between US mpg, imperial mpg, litres per 100 km and kilometres per litre.

Draft — figures checked, awaiting an editorial read. Not indexed.

Formula: L/100 km = 235.2146 ÷ mpg (US). The relationship is reciprocal, not a simple factor.

What this calculates

This converts fuel economy between the four systems in common use, and it is genuinely different from every other converter here because the relationship is reciprocal rather than linear. Miles per gallon measures distance per unit of fuel; litres per 100 kilometres measures fuel per unit of distance. They are inverses, so as one rises the other falls, and you cannot convert between them by multiplying by a constant. Getting this wrong produces answers that are not merely inaccurate but backwards.

How it works

Everything routes through US mpg. The conversion to litres per 100 km is 235.2146 divided by the mpg figure — that constant comes from combining the miles-to-kilometres and gallons-to-litres factors and multiplying by 100. Imperial mpg is a straight factor away from US mpg, because the imperial gallon is 1.2009 times the US gallon; this is why British car figures have always looked about 20% better than American ones for the same vehicle. Kilometres per litre is a linear factor from mpg since both are distance-per-fuel.

Worked example

A car rated 30 mpg (US): 235.2146 ÷ 30 = 7.84 L/100 km. The same car quoted in imperial mpg is 30 × 1.2009 = 36.0 mpg, which sounds noticeably more efficient and is exactly the same car. The reciprocal relationship shows up clearly at the extremes: improving from 15 to 20 mpg saves 5.2 L/100 km, while improving from 40 to 45 mpg saves only 0.65 — far less, for the same 5 mpg gain.

Common mistakes

Treating the conversion as linear, which is the fundamental error. Second, comparing US and imperial mpg figures directly — a 20% difference that makes older British road tests look far better than they were. Third, and more subtly, using mpg to reason about fuel savings at all: because it is reciprocal, equal mpg improvements are not equal fuel savings, which is precisely why regulators outside the US prefer L/100 km.

Common questions

What is 30 mpg in L/100 km?

7.84 litres per 100 km. The conversion is 235.2146 ÷ mpg, not a multiplication, because the two units are reciprocals of each other.

Why is imperial mpg higher than US mpg?

Because an imperial gallon is 4.546 litres against the US gallon's 3.785 — about 20% larger. The same car travels further on the bigger gallon, so the number is higher without the efficiency changing.