Understanding Fuel Economy: MPG vs L/100km

Fuel economy gets measured two fundamentally different ways depending on where you are: MPG counts distance per unit of fuel, while L/100km counts fuel per unit of distance, and the two don’t convert with a simple multiplication.

MPG vs L/100km

Miles per gallon (MPG) tells you how far a vehicle travels on a fixed amount of fuel, more MPG is better. Liters per 100 kilometers (L/100km) tells you how much fuel a vehicle burns over a fixed distance, less L/100km is better. Because one measures distance-per-fuel and the other measures fuel-per-distance, they’re inversely related, not directly proportional.

US MPG vs UK (Imperial) MPG

Just as with the US and UK gallon covered in the Volume guide, a US MPG figure and a UK MPG figure for the exact same fuel efficiency are different numbers, because they’re built on different-sized gallons. A UK imperial gallon is larger, so the same real-world efficiency produces a higher UK MPG number than a US MPG number.

The Conversion Formula

Worked Example

Common Mistakes

• Averaging MPG values directly. Because MPG and fuel consumption are inversely related, averaging two MPG figures doesn’t give the correct average fuel consumption; averaging the L/100km (or gallons-per-mile) figures instead does.
• Mixing up US and UK MPG. The same vehicle can have noticeably different “MPG” numbers quoted depending on which gallon definition is being used.
• Assuming higher MPG always means proportionally less fuel used. Going from 20 to 40 MPG halves fuel consumption per mile, but going from 40 to 60 MPG saves proportionally less, since the relationship is inverse, not linear.

Use the Fuel Economy Converter

The Fuel Economy Converter converts between MPG (US and UK) and L/100km instantly.