Turning a consumption figure into a range figure
A European brochure says a car uses 5.4 litres per 100 kilometres. That is a consumption figure: fuel per fixed distance. An American or British reader wants a range figure: distance per fixed fuel. The conversion between them is a reciprocal, which is why this page divides a constant by your number rather than multiplying.
Because it is a reciprocal, the scale is not linear. Halving L/100km doubles MPG, but shaving one litre off the figure is worth a different number of MPG depending on where you start. Going from 10 to 9 L/100km gains 2.6 mpg (US); going from 5 to 4 gains 11.8 mpg. This is the biggest practical difference between the two systems and the guide on which unit is clearer goes into it.
Two formulas, one for each gallon
MPG (US) = 235.214583 ÷ L/100km
MPG (UK) = 282.480936 ÷ L/100km
The US constant is 100 times 3.785411784 (litres per US gallon) over 1.609344 (kilometres per mile). The UK constant swaps in 4.54609 litres for the imperial gallon. Both gallons and the mile are exact legal definitions, so the constants are exact to the number of decimals shown and the converter carries more than that internally.
Because the imperial gallon is larger, the same L/100km always gives a UK mpg figure 20.09% higher than the US one. That ratio, 1.200950, is fixed; it does not depend on the car.
A worked example, both ways
A diesel estate rated at 6.5 L/100km on the WLTP combined cycle. For a US reader: 235.214583 divided by 6.5 is 36.19 mpg. For a UK reader: 282.480936 divided by 6.5 is 43.46 mpg. Same car, same tank, two honest answers, because the gallon in each country is a different size.
This is the single most common confusion in fuel economy. A British reader who sees 36 mpg quoted for a car they know as a 43 mpg car assumes a mistake. There is no mistake, just two gallons.
Where the L/100km figure comes from
Since 2018 every new car sold in the EU has been tested on WLTP, and the figure on the brochure is the combined result of that test. Canada uses a five-cycle test based on the EPA method but publishes the result in L/100km. Australia and New Zealand use their own ADR 81/02 cycle, still in L/100km. None of these are identical, so a converted mpg figure tells you the efficiency on that country's test, not on the EPA's.
Whichever spelling you prefer, litres or liters, the unit is the same, and however it is abbreviated, l/100km, ltr/100km, litres per 100 or l/km, it converts the same way. The distinction that actually changes the answer is the gallon, not the spelling.
Common errors
- Reading a UK mpg figure as a US one, or the reverse. Always check which gallon a source uses. American sources use US gallons; British ones use imperial.
- Using the highway or extra-urban figure when you meant combined.
- Converting L/100km to km/L by dividing by 100 the wrong way round. km/L is 100 divided by L/100km, not the other way; there is a km/L converter if that is the unit you need.
- Expecting the EPA figure to match a converted WLTP figure. Different test, different number.