MPG vs L/100km: How to Compare Fuel Economy
Fuel economy is quoted two different ways, and they run in opposite directions: with miles per gallon, higher is better, while with litres per 100 km, lower is better. That inversion causes real confusion when comparing cars across regions. Here is how to line them up.
Two scales, opposite directions
MPG measures distance per fuel — how far you go on a fixed amount, so more is better. L/100km measures fuel per distance — how much you burn to cover a fixed distance, so less is better. They describe the same thing from opposite ends, which is why you cannot just compare the numbers directly.
Converting between them
Because they are inverses, you divide rather than multiply: L/100km = 235.2 / MPG (using US gallons). A 30 MPG car is 235.2 / 30 = about 7.8 L/100km; a 40 MPG car is about 5.9. The Fuel Economy Converter does it both ways.
The gallon trap
There are two different gallons: the US gallon (3.785 litres) and the larger imperial gallon (4.546 litres) used in the UK. The same car reads as a higher MPG in imperial gallons than in US gallons, purely because the gallon is bigger. Always check which gallon a figure uses before comparing across the Atlantic.
Why the inversion matters
Because the scale is inverted, equal MPG gains are not equal fuel savings. Going from 10 to 20 MPG saves far more fuel than going from 40 to 50 MPG, even though both are a 10 MPG jump — the biggest savings are at the thirsty end. L/100km makes this obvious; MPG hides it.
Frequently asked questions
Which is better, high MPG or low L/100km? Both describe efficiency — high MPG and low L/100km are the same good outcome.
Why does my UK car seem more efficient? Likely the imperial gallon, which is about 20% larger than the US one.
Is L/100km used in the US? Rarely — the US uses MPG, while most of the rest of the world uses L/100km.
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Last updated: August 27, 2026