This is the one converter on this site that is not a simple multiply-by-a-factor relationship, and that is the entire reason it exists as its own page rather than a row in a general conversion table.
Why fuel economy does not convert like everything else
Miles per gallon measures distance per volume — a bigger number means better economy. Litres per 100 kilometres measures the opposite: volume per distance — a smaller number means better economy. Converting between them means inverting the relationship, not multiplying by a fixed factor, which is exactly why an intuitive shortcut like “just average the two figures” produces a wrong answer for fuel economy specifically, even though the same shortcut works fine for length or weight.
How it works
L/100km from mpg
L/100km = (100 × gallon in litres) ÷ (mpg × mile in km)
Using the exact definitions already used elsewhere on this site — 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L — the US constant works out to 235.214583…, and the same formula run with the imperial gallon (4.54609 L) gives 282.480936…. The same division runs in reverse to go from L/100km back to mpg.
US and UK “mpg” are not the same unit
The two gallons differ by about 20%, so a car quoted at “40 mpg” in a UK review and “40 mpg” in a US one are not achieving the same real-world efficiency — the UK figure describes roughly 17% better efficiency, because the imperial gallon is the larger of the two. Comparing a US-market car’s mpg directly against a UK-market car’s mpg, without converting, compares two different units that happen to share a name.
How to use this calculator
- Enter a figure in whichever unit you have — mpg (US), mpg (UK) or L/100km.
- The others update, correctly inverted rather than linearly scaled.
- If comparing cars from different markets, convert both to the same unit first — never compare a US mpg figure to a UK mpg figure directly.
Frequently asked questions
Why can’t I just average two mpg figures to get a combined rating?
Because mpg is a rate measured in the wrong direction for simple averaging — the mathematically correct way to combine fuel economy figures over equal distances is a harmonic mean, not an arithmetic one, precisely because mpg is distance-per-volume rather than volume-per-distance.
Is a bigger or smaller L/100km number better?
Smaller is better for L/100km — it means less fuel used to cover the same distance. This is the opposite direction from mpg, where a bigger number is better, which is exactly the source of the conversion trap this calculator exists to solve.
Why are US and UK mpg different for the same car?
Because the two gallons are different sizes. The US gallon is about 3.785 litres; the imperial gallon is about 4.546 litres, roughly 20% larger — so “miles per gallon” measures against a bigger unit of fuel in the UK figure, making the same real efficiency look like a bigger mpg number.
What does km/L mean, and how does it relate to L/100km?
Kilometres per litre is the metric equivalent of mpg — bigger is better, same direction. It is the reciprocal of L/100km scaled by 100, so the two metric figures convert to each other by simple division, unlike the mpg-to-L/100km relationship.
