Fuel Economy: L/100km ↔ mpg
Litres per hundred kilometres to miles per gallon and back — with US and UK gallons kept separate, because they are not the same.
- L/100km
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- km/L
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- mpg US
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- mpg UK
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Lower is better for L/100km; higher is better for the other three. The two mpg figures differ because a UK gallon is a fifth bigger than a US one.
Fast, easy and free — your answer appears as you type, and whatever you enter stays on your device.
Enter fuel consumption in any of the four common measures — L/100km, km/L, US mpg or UK mpg — and read all of them at once. These convert by reciprocal, not by multiplying, which is why ordinary unit converters get fuel economy wrong.
How to use it
- Enter what you knowThe figure from the car ad, the dashboard or the rental agreement, and which measure it is in.
- Read all fourEvery measure updates together — no direction to choose.
- Mind the directionLower is better in L/100km; higher is better in the rest. The note under the numbers keeps that straight.
The conversion that flips
Fuel economy is the odd one out among conversions. Every other unit pair on this site converts by a single multiplication; these convert by reciprocal, because Europe measures fuel per distance and the anglophone world measures distance per fuel. The consequence is unintuitive: relationships invert. A small improvement in a thirsty car's mpg is a large saving in litres; a big mpg jump in an already-efficient car is pennies. Comparing cars in L/100km avoids the illusion entirely, which is why this tool shows all four measures simultaneously instead of asking you to pick a direction.
Two gallons, one word
The US gallon is 3.785 litres; the imperial gallon is 4.546. The same car therefore scores 20% "better" in UK mpg than US mpg — 40 mpg US is 48 mpg UK, identical consumption. Car reviews, rental paperwork and forum advice rarely say which gallon they mean, and a cross-Atlantic comparison made on the wrong one is off by exactly that fifth. Both figures are printed here, labelled, every time.
Figures you enter stay on your device, like everything on Eliroo.
Questions people ask
What is 7.5 L/100km in mpg?
About 31.4 mpg US, or 37.7 mpg UK. The two disagree because the gallons differ by 20% — always check which gallon a figure means before comparing cars across markets. It is the single most common fuel-economy mistake.
Why can't a normal unit converter do this?
Because the measures point in opposite directions: L/100km counts fuel per distance, mpg counts distance per fuel. Converting means taking a reciprocal — mpg US = 235.21 ÷ L/100km — not multiplying by a factor. Double the mpg and you halve the L/100km; a linear converter cannot express that.
Which measure is best?
For comparing costs, L/100km — fuel used scales directly with money spent, and differences mean what they seem. mpg misleads at the top: improving 15→20 mpg saves far more fuel over a year than 40→50 mpg, though the second jump sounds bigger. This asymmetry has a name in the research: the mpg illusion.
What counts as good fuel economy?
Rough 2020s petrol-car anchors: under 5 L/100km (47+ mpg US) is excellent, 6–8 is ordinary, above 10 (below 23.5 mpg US) is thirsty. Hybrids commonly run 4–5; the figure on the ad is a lab cycle, and real driving typically lands 10–20% worse.
How do I work out fuel cost per trip?
L/100km makes it direct: consumption × distance ÷ 100 × price per litre. A 450 km trip at 7.5 L/100km and 1.90/L: 7.5 × 4.5 × 1.9 ≈ 64. No other measure computes cost that cleanly, which is another argument for it.
Is anything stored?
No — the arithmetic runs on your device, offline once loaded.