Fuel Economy Conversion Formula
MPG and L/100km are inversely related — flip and scale to convert.
L/100km = 235.215 ÷ MPG (US)
How it works
Miles per gallon and liters per 100 kilometers measure the same thing from opposite ends. MPG is distance per unit of fuel, so bigger is better. L/100km is fuel per unit of distance, so smaller is better. Because one is the reciprocal of the other, you cannot convert between them by adding or scaling - you have to divide. The constant 235.215 is not arbitrary: 100 kilometers is 62.1371 miles, and a US gallon is 3.785412 liters, so 62.1371 x 3.785412 = 235.215. Use 282.481 instead if the figure is in imperial gallons, because an imperial gallon is 4.54609 liters. This is the single most common mistake in fuel-economy conversion: a UK MPG figure run through the US constant comes out about 17 percent lower than it should, making the car look considerably more economical than it is.
Variables
- MPGMiles per US gallon
- L/100kmLiters per 100 kilometers
Worked examples
| Input | Result |
|---|---|
| 30 MPG | ≈ 7.84 L/100km |
| 50 MPG | ≈ 4.70 L/100km |
Practical applications
- Reading a European car review that quotes L/100km when you think in MPG
- Comparing a US-market and UK-market version of the same model, where both are quoted in MPG but on different gallons
- Working out running costs for a fleet where some vehicles are logged metric and some imperial
Common mistakes
- Treating the relationship as linear. Going from 20 to 40 MPG saves far more fuel than going from 40 to 60, even though both are a 20 MPG improvement.
- Using the US constant on an imperial MPG figure, or the reverse. An imperial gallon is about 20 percent larger than a US gallon.
- Averaging MPG figures directly. To average fuel economy across trips you must average the fuel used per distance, not the MPG values.
Use it in a tool
FAQs
Why is the relationship not linear?+
MPG measures distance per fuel; L/100km measures fuel per distance. They're reciprocals — doubling MPG halves L/100km.