Fuel Efficiency Converter
Convert fuel efficiency between km/L, L/100km, mpg (US), and mpg (UK).
Lower L/100km and higher mpg mean better economy.
Enter any value to convert.
Type in any field — the others update instantly. Note: L/100km is an inverse relationship.
Fuel efficiency can be measured two opposite ways, and the two don’t move in the same direction, which is the source of endless confusion. Distance per fuel (mpg, km/L) goes up as a car gets thriftier; fuel per distance (L/100km) goes down. They describe the same thing inside out.
Key conversions:
- L/100km = 100 ÷ (km/L) (an inverse relationship)
- km/L to mpg (US) = km/L × 2.35215
- km/L to mpg (UK) = km/L × 2.82481
For a sense of scale:
- An efficient car: 5 L/100km (47 mpg US)
- An average car: 8 L/100km (29 mpg US)
- An SUV or truck: 12 L/100km (20 mpg US)
- A hybrid: 4 L/100km (59 mpg US)
The big trap is comparing US and UK mpg figures as if they were the same. They aren’t, because the Imperial gallon is about 20 percent larger than the US gallon. A car rated “40 mpg” in Britain is really about 33 mpg in American terms. Always check which gallon a number uses before comparing cars across the Atlantic.
The inverse nature of L/100km also makes savings deceptive. Dropping from 15 to 12 L/100km saves more actual fuel than dropping from 8 to 6, even though the second looks like the bigger improvement, because the gains shrink as efficiency climbs.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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