Fuel Economy Converter
Convert fuel economy between miles per gallon (US and UK), kilometres per litre and litres per 100 km — the one conversion that runs backwards.
How it works
This family is different from every other converter here, because the units run in opposite directions. Miles per gallon and km/l measure distance per fuel — higher is better. Litres per 100 km measures fuel per distance — lower is better. Converting between them means taking a reciprocal, not multiplying.
That inversion has a consequence people miss: equal mpg gains are not equal fuel savings. Going from 10 to 15 mpg saves far more fuel over the same distance than going from 30 to 35, even though both are a 5 mpg improvement. L/100km makes that obvious, which is why most of the world uses it.
US and UK gallons differ by about 20%, so mpg figures are not comparable between them. A car rated 40 mpg in the UK is about 33 mpg in US terms — the same car, a different gallon.
The formula
mpg (US) → L/100km
L/100km = 235.215 ÷ mpg
mpg (UK) → L/100km
L/100km = 282.481 ÷ mpg
km/L → L/100km
L/100km = 100 ÷ km/L
Worked examples
| Scenario | Working | Result |
|---|---|---|
| 30 mpg (US) to L/100km | 235.215 ÷ 30 | 7.84 L/100km |
| 40 mpg UK vs US | Different gallon sizes | ≈ 33.3 mpg US |
| 10 → 15 mpg vs 30 → 35 mpg | Reciprocals | The first saves far more fuel |
When you'd use it
- Comparing a car's economy against a figure quoted abroad
- Converting a manufacturer's rating to your local unit
- Working out running costs per 100 km
- Comparing US and UK mpg figures honestly
Common questions
Why can't I just multiply to convert mpg to L/100km?
Because they are inverse measures. Mpg is distance per fuel, L/100km is fuel per distance, so the conversion is a division: 235.215 ÷ mpg for US gallons. Multiplying gives a meaningless number.
Is US mpg the same as UK mpg?
No. A UK gallon is about 20% larger, so the same car scores about 20% higher in UK mpg. A 40 mpg UK figure is roughly 33 mpg US — always check which gallon a quoted figure uses.
Why does the rest of the world use L/100km?
Because it scales linearly with what you actually spend. Halving L/100km halves your fuel bill; a 5 mpg improvement means something completely different at 10 mpg than at 40 mpg. L/100km makes fuel cost easy to compare.
Is anything I enter sent anywhere?
No. Every conversion runs in your browser, so nothing you type is uploaded, logged or stored. There is no account and no record of what you converted.