L/100 km and mpg: converting fuel economy
To go from US miles per gallon to L/100 km, divide 235.215 by the mpg figure: 30 mpg = 7.84 L/100 km. The same division works the other way: 8 L/100 km = 29.4 mpg.
Updated November 6, 2026 · Sources
Two measures that work backwards
L/100 km says how much fuel a distance takes: lower is better. Mpg says how far one gallon takes you: higher is better.
That is why you do not multiply: you divide a constant by the figure. With the US gallon the constant is 235.215; with the imperial gallon, 282.481.
Where the constant comes from
- 1 mile = 1.609344 km.
- 1 US gallon = 231 cubic inches, with 1 inch = 2.54 cm, so 3.785411784 L.
- 1 imperial (UK) gallon = 4.54609 L.
- Constant = 100 × liters per gallon ÷ kilometers per mile.
Conversion table
8 L/100 km is 29.4 US mpg but 35.3 imperial mpg: always say which gallon you mean.
| Consumption | mpg (US) | mpg (UK) |
|---|---|---|
| 4 L/100 km | 58.8 | 70.6 |
| 5 L/100 km | 47 | 56.5 |
| 6 L/100 km | 39.2 | 47.1 |
| 7 L/100 km | 33.6 | 40.4 |
| 8 L/100 km | 29.4 | 35.3 |
| 10 L/100 km | 23.5 | 28.2 |
| 12 L/100 km | 19.6 | 23.5 |
| 15 L/100 km | 15.7 | 18.8 |
The mpg trap
Over 100 miles, going from 10 to 20 mpg saves 5 gallons. Going from 30 to 50 mpg saves only 1.33, even though the jump looks bigger.
To compare fuel savings, L/100 km is clearer: the figures subtract directly.