Fuel Economy Converter (MPG ↔ L/100km ↔ km/L)
Convert fuel economy between US mpg, UK mpg, liters per 100km, and km per liter — unlike most unit converters, these aren't a simple multiply.
- Converted value
- 7.8405
How it works
Most of this site's unit converters just multiply by a fixed constant, because both units measure the same kind of ratio. Fuel economy is different: mpg and km/L measure distance per unit of fuel (bigger is more efficient), while liters per 100km measures fuel per unit of distance (smaller is more efficient) — they're inverses of each other, not just rescaled copies.
Every value is first converted to liters per 100km, the calculator's common base unit: for mpg figures, L/100km = (100 × liters per gallon) ÷ (mpg × 1.609344 km per mile), using the exact US gallon (3.785411784 L) or UK/imperial gallon (4.54609 L) depending on which mpg you chose; for km/L it's simply 100 ÷ (km/L). Converting from L/100km to any target unit solves that same relationship for the target — dividing, not multiplying, which is why doubling a mpg value roughly halves its L/100km equivalent rather than doubling it too.
US and UK/imperial gallons are different sizes (the imperial gallon is about 20% larger), so the same mpg number means a different real fuel economy depending on which gallon it refers to — this calculator keeps them as two distinct units rather than a single generic 'mpg' to avoid that mismatch.
FAQ
Why does a higher mpg convert to a lower L/100km?
They measure fuel economy in opposite directions — mpg is how far you get per unit of fuel (higher is better), while L/100km is how much fuel a fixed distance takes (lower is better). Converting between them involves a division, not a multiplication, which is why the relationship is inverse rather than proportional.
Why are there separate US and UK mpg options?
A US gallon (3.785411784 liters) and a UK/imperial gallon (4.54609 liters) are different volumes, so 'miles per gallon' means a meaningfully different fuel economy depending on which one is meant — a vehicle rated at 40 US mpg is actually more fuel-efficient than one rated at 40 UK mpg, since the US gallon is smaller.