Guide · Published October 26, 2026 · By the site maintainer
MPG vs. liters per 100 km: convert fuel economy both ways
Fuel economy is an inverse conversion: divide 235.215 by the US mpg figure to get liters per 100 km, or divide 235.215 by the L/100 km figure to get US mpg. Higher mpg is better; lower L/100 km is better. The two scales run in opposite directions, which is why you cannot convert them with a simple multiply.
Miles per gallon measures distance per unit of fuel. Liters per 100 km measures fuel per unit of distance. They describe the same efficiency from opposite ends, so the conversion is a division into a constant rather than a multiplication by a factor. That constant packs in both the mile-to-kilometer and gallon-to-liter conversions at once.
The formulas
- US mpg → L/100 km: L/100 km = 235.215 ÷ mpg (US)
- L/100 km → US mpg: mpg (US) = 235.215 ÷ L/100 km
- Imperial (UK) mpg → L/100 km: L/100 km = 282.481 ÷ mpg (UK)
- L/100 km → Imperial mpg: mpg (UK) = 282.481 ÷ L/100 km
The constants differ because the gallons differ. A US gallon is 3.78541 liters; an Imperial gallon is 4.54609 liters. Everything else — the 100 km reference distance and the mile — is the same, which is why the UK constant is larger by exactly the ratio of the two gallons. Our US customary vs. Imperial guide covers the other places the two systems diverge.
Conversion table
| US mpg | L/100 km | Imperial mpg |
|---|---|---|
| 15 mpg | 15.68 L/100 km | 18.0 mpg |
| 20 mpg | 11.76 L/100 km | 24.0 mpg |
| 25 mpg | 9.41 L/100 km | 30.0 mpg |
| 30 mpg | 7.84 L/100 km | 36.0 mpg |
| 35 mpg | 6.72 L/100 km | 42.0 mpg |
| 40 mpg | 5.88 L/100 km | 48.0 mpg |
| 45 mpg | 5.23 L/100 km | 54.0 mpg |
| 50 mpg | 4.70 L/100 km | 60.0 mpg |
| 55 mpg | 4.28 L/100 km | 66.1 mpg |
| 60 mpg | 3.92 L/100 km | 72.1 mpg |
Two anchors cover most real cars. A car using 5.0 L/100 km gets 47.0 mpg US (56.5 mpg UK) — genuinely frugal. A car at 8.0 L/100 km gets 29.4 mpg US, about average for a gasoline sedan. Anything above 12 L/100 km, under 20 mpg US, is thirsty.
The Imperial mpg trap
Because the Imperial gallon is about 20 percent larger, the same car always shows a bigger mpg number in Britain than in America. A car rated 40 mpg in the UK is a 33.3 mpg car in the US — identical efficiency, different gallon. Comparing a UK review's mpg figure directly against a US window sticker makes the British car look a fifth more efficient than it is. Convert through L/100 km, which has no national variants, whenever a comparison crosses the Atlantic.
Canada adds one more wrinkle: fuel is sold in liters and consumption is quoted in L/100 km, but older Canadian ratings and cross-border advertising still use mpg, sometimes US and sometimes Imperial. The L/100 km figure is the only one that cannot be misread.
A worked rental-car example
You rent a car in Spain whose display reads 6.5 L/100 km, and your trip is 900 km. Fuel used: 6.5 × 9 = 58.5 liters, which the liters and gallons converter turns into 15.5 US gallons. In mpg terms the car is returning 235.215 ÷ 6.5 = 36.2 mpg US — respectable for a highway trip. And if the speedometer is the number that worries you rather than the fuel, the mph and km/h converter covers the limits side of the same drive.
Why the difference between 30 and 35 mpg is small
Because mpg is a reciprocal scale, equal steps in mpg are not equal savings. Over 100 miles, improving from 15 to 20 mpg saves 1.67 gallons; improving from 35 to 40 mpg saves only 0.36 gallons. The L/100 km scale shows this honestly: 15.68 falling to 11.76 is a big drop, while 6.72 falling to 5.88 is a small one. When you compare cars, convert both to L/100 km (or gallons per 100 miles) before deciding how much an efficiency upgrade is worth.
Whichever unit your dashboard uses, the method is the same: fill the tank, note the distance, and divide — distance per fuel for mpg, fuel per distance for L/100 km. One honest tank of your own driving beats any converted brochure figure.