MPG vs L/100km: Comparing Fuel Economy Across Regions
Fuel economy is the one conversion where the two units run in opposite directions. MPG goes up as a car gets more efficient. L/100km goes down. So a bigger number is better in one and worse in the other, which makes eyeballing a comparison impossible. The Fuel Economy Converter handles it.
Why they are inverses
Miles per gallon is distance per unit of fuel. Litres per 100 km is fuel per unit of distance. They are reciprocals of each other, which is why the conversion is a division rather than a multiplication: 235.2 divided by the MPG gives L/100km, and the same division goes back the other way.
So 30 MPG is 7.8 L/100km, and 40 MPG is 5.9. Notice the asymmetry: going from 30 to 40 MPG saves 1.9 litres per 100 km, but going from 50 to 60 MPG saves only 0.8. Equal steps in MPG are not equal savings in fuel, which is the practical argument for the European unit - it tells you directly what you will spend.
The gallon trap
That 235.2 figure assumes a US gallon. For an imperial gallon it is 282.5, because an imperial gallon is about twenty per cent larger.
Which means a British car doing 40 MPG and an American car doing 40 MPG are not equally efficient. The British figure is 7.1 L/100km and the American one 5.9 - the British car is genuinely using more fuel for the same quoted number. Any cross-Atlantic comparison that ignores this flatters the British car by a fifth, and the Volume Converter is the place to see why the gallons differ.
Canada uses L/100km, the US uses MPG with US gallons, and the UK uses MPG with imperial gallons, so all three of the awkward cases are live.
km/L, and the official figures
Some countries, Japan among them, use kilometres per litre, which runs the same direction as MPG. Multiply km/L by 2.35 for US MPG or 2.82 for imperial.
Worth knowing about any of these numbers: official economy figures come from a standardised test, not from driving. Real-world consumption is routinely ten to thirty per cent worse, and the gap is widest for hybrids, whose test cycle flatters them. The figure is useful for comparing cars with each other and not for predicting your fuel bill.
Common questions
What is the formula?
235.2 divided by MPG for US gallons, 282.5 for imperial. The same division converts back.
Which unit is better?
L/100km, because it is proportional to what you actually spend. MPG hides diminishing returns.
Why is my real economy worse than the sticker?
Short trips, cold starts, speed and air conditioning all cost fuel, and none of them feature much in the test cycle.
How do I compare an electric car?
Different units entirely - miles per kWh, or kWh per 100 km. There is no fuel to measure.
Got an economy figure to convert?