Why the Same Car Gets a Different MPG in Britain

An identical car, driven identically, reports a meaningfully higher "miles per gallon" figure in the UK than in the US. Nothing about the car changes — what changes is the definition of a gallon, which is one of those unit mismatches that looks like it should be a rounding error and is actually a 20% difference.

Two different gallons

The US gallon and the imperial (UK) gallon were never the same unit — they're separate historical definitions that happened to keep the same name. A US gallon is about 3.785 liters; an imperial gallon is about 4.546 liters, roughly 20% larger. "Miles per gallon" divides distance by fuel volume, so measuring the identical fuel volume against a larger denominator unit (the bigger imperial gallon) produces a smaller "gallons used" figure — and dividing miles by a smaller number of gallons produces a larger mpg number. A car that gets 30 mpg in the US reports about 36 mpg under the UK definition, purely from the unit difference, with zero change in actual fuel efficiency.

Why this trips people up more than most unit conversions

Most unit mismatches (miles vs. kilometers, Fahrenheit vs. Celsius) are obviously different scales — nobody expects 30 miles to equal 30 kilometers. "Miles per gallon" is more deceptive because both countries use the identical phrase and the identical abbreviation, "mpg," for numbers that mean something different. Comparing a US car review's mpg figure directly against a UK car review's mpg figure without converting is a genuinely common mistake, and it makes the UK car look meaningfully more efficient than it actually is relative to the US one.

Liters per 100km flips the comparison entirely

Most of the rest of the world doesn't use "distance per fuel volume" at all — it uses liters per 100 kilometers, which measures fuel consumed over a fixed distance instead. That inverts the usual intuition: with mpg and km/L, a bigger number means better efficiency, but with L/100km, a smaller number means better efficiency, since it's literally "how much fuel did this car burn to cover a fixed distance." A car improving from 8 L/100km to 6 L/100km is getting more efficient even though the number went down — the opposite direction from an mpg improvement, which is exactly the kind of scale confusion that trips up cross-border efficiency comparisons.

Why one canonical unit makes the conversion reliable

With four units in play (US mpg, UK mpg, km/L, L/100km) and two possible scale directions, converting pairwise between all six combinations directly would require six separate formulas, each a chance to introduce an error. Converting every value through a single canonical unit first — kilometers per liter, in this case — cuts that down to just two conversions per unit (into the canonical unit, and back out), which is also exactly how a gas mileage calculator handles all four at once without the combinatorics getting out of hand.