Fuel Consumption Converter

Convert between mpg, L/100km, and km/L, with why these two systems measure fuel economy in opposite directions explained.

Why mpg and L/100km Measure the Same Thing Backwards

Fuel consumption is one of the few conversion categories where the two major systems don’t just use different units, they actually measure the underlying relationship in opposite directions, which is exactly why converting between them isn’t a simple multiplication the way most conversions are. Miles per gallon expresses how far a vehicle travels on a fixed amount of fuel, meaning a higher number is better, while litres per hundred kilometres expresses how much fuel a vehicle consumes over a fixed distance, meaning a lower number is better, an inverse relationship that means converting from one to the other requires dividing into a constant rather than simply applying a ratio. This inversion causes a genuinely counterintuitive effect: a fixed improvement in mpg represents a shrinking real-world fuel saving as the starting mpg figure gets higher, since going from 10 to 20 mpg saves considerably more fuel over the same distance than going from 40 to 50 mpg does, even though both represent the same ten-mpg improvement, a distortion that disappears entirely once the same comparison is made in litres per hundred kilometres instead, which is part of why fuel economy researchers generally consider the consumption-based metric more meaningful for genuinely comparing real-world fuel savings.