Which mpg does my car's display use?

A car sold in the United States shows US mpg; one sold in the United Kingdom shows imperial. If a figure looks unexpectedly good for the car, it is probably imperial — the same vehicle reads about 20% higher in UK mpg than in US mpg.

Is L/100km better than mpg?

For arithmetic, yes, and that is not just preference. Fuel per distance adds and averages correctly, scales linearly with journey length, and answers the question you usually have — how much fuel will this trip take. Distance per fuel inverts all of that, which is where the traps come from.

Fuel Economy Converter – MPG to L/100km and km/L

Fuel economy converter

Convert between litres per 100 km, miles per gallon and kilometres per litre.

Value

From unit

Litres per 100 km (L/100km)	7
Miles per gallon (US)	33.602
Miles per gallon (imperial)	40.354
Kilometres per litre (km/L)	14.286

These are reciprocals, not scalings: litres per 100 km measures fuel per distance, miles per gallon measures distance per fuel. Halving your consumption doubles your mpg, and averaging two mpg figures gives the wrong answer.

Why you cannot average miles per gallon

Litres per 100 km and miles per gallon measure the same thing upside down: one is fuel per distance, the other distance per fuel. Because they are reciprocals rather than rescalings, the arithmetic behaves in ways that catch people out — most importantly, you cannot average mpg figures, and equal-looking improvements are not equal.

How it works

Converts between L/100km, US mpg, imperial mpg and km/L, all through kilometres per litre.
Keeps US and imperial gallons apart — an imperial gallon is 20% larger, so the same car reads about 20% better in UK mpg.
Shows all four figures at once, which makes the reciprocal relationship visible rather than theoretical.

km per litre is the pivot

  mpg (US)  -> km/L   x 1.609344 / 3.785411784
  mpg (UK)  -> km/L   x 1.609344 / 4.54609
  L/100km   -> km/L   100 / value        <- note the reciprocal

the shortcuts that follow
  L/100km = 235.214583 / mpg (US)
  L/100km = 282.480936 / mpg (UK)

Worked example

A car using 7 litres per 100 km, in both mpg conventions.

start 7 L/100km
to km per litre 100 / 7 = 14.2857 km/L
into US gallons 14.2857 x 3.785411784 / 1.609344
result 33.6 mpg (US)
the same in imperial 40.4 mpg (UK)

7 L/100km is 33.6 US mpg or 40.4 imperial mpg. The same car, the same fuel, two numbers 20% apart — which is why a US and a UK review of the same model appear to disagree.

Reading the result

You cannot average mpg. Drive 100 miles at 10 mpg and 100 at 50 mpg and the average is not 30 — you burned 10 gallons and 2 gallons for 200 miles, so it is 200/12 = 16.7 mpg. Averaging works fine in L/100km, because fuel per distance adds up linearly and distance per fuel does not.
Improvements are worth more at the bottom. Going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50, even though the second looks like a bigger jump. In L/100km the same fact is obvious: 15.7 to 11.8 saves 3.9 litres per 100 km, while 5.9 to 4.7 saves only 1.2.
An imperial gallon is 4.54609 litres against the US gallon's 3.785411784 — about 20% larger. Neither is a rounding of the other; both are defined exactly, and this is why a converter must ask which one you mean rather than assuming.

Common questions

Which mpg does my car's display use?: A car sold in the United States shows US mpg; one sold in the United Kingdom shows imperial. If a figure looks unexpectedly good for the car, it is probably imperial — the same vehicle reads about 20% higher in UK mpg than in US mpg.
Is L/100km better than mpg?: For arithmetic, yes, and that is not just preference. Fuel per distance adds and averages correctly, scales linearly with journey length, and answers the question you usually have — how much fuel will this trip take. Distance per fuel inverts all of that, which is where the traps come from.