Why this conversion makes a car look better
A number in km/L becomes a bigger number in MPG, always. The US gallon is 3.785 litres and the mile is 1.609 km, so one kilometre per litre is 2.352 miles per gallon, and an imperial mpg is bigger still at 2.825. A hatchback that does 18 km/L is a 42 mpg car in America and a 51 mpg car in Britain.
Nothing about the car has changed; the units are simply larger. Readers in India sometimes react to a 45 mpg British review with disbelief, then find the car is a 16 km/L car they already know. It is worth keeping the factor in your head when reading foreign road tests: a bit more than double for US mpg, a bit less than triple for UK mpg.
The formula
MPG (US) = km/L ÷ 0.425144 (or km/L × 2.352146)
MPG (UK) = km/L ÷ 0.354006 (or km/L × 2.824809)
The 0.425144 is 1.609344 over 3.785411784, and 0.354006 is 1.609344 over 4.54609. The converter divides by the full-precision factor; the multiplied forms are shown for readers who prefer to work that way and give the same result.
To go to L/100km instead, divide 100 by the km/L figure. 15 km/L is 6.67 L/100km. The L/100km to MPG page picks up from there if you need both.
A worked example from an Indian spec sheet
A petrol hatchback is certified by ARAI at 20.5 kmpl. Divide by 0.425144 and you get 48.2 mpg (US); divide by 0.354006 and you get 57.9 mpg (UK). Those are strong figures, and they should be read with the ARAI test in mind: it is a gentle cycle and real-world results from owners are commonly 15 to 25% lower.
Take a more typical owner's figure instead: 15 km/L measured at the pump over a month. That is 35.3 mpg (US) or 42.4 mpg (UK), and it is the sort of number the manual calculation guide shows you how to measure properly.
Who uses km/L
India (as kmpl), Japan, Brazil, Indonesia, Thailand, the Philippines, Pakistan, Bangladesh and the Gulf states all quote fuel economy in kilometres per litre, whether officially or in everyday use. It is arguably the most widely spoken fuel economy unit in the world by population, even though L/100km is the official unit in more countries.
Litres or liters, the spelling varies by country but the litre itself does not. The unit is one cubic decimetre everywhere.
Where people go wrong
- Multiplying by 0.425 instead of dividing. That turns 15 km/L into 6.4 mpg, which is obviously wrong, but the same slip with less obvious numbers goes unnoticed.
- Assuming a km/L figure from a motorcycle converts to something comparable with a car. The maths works; the comparison does not mean much.
- Taking an ARAI or JC08 figure as a real-world one. Both cycles run cooler and slower than daily driving.
- Quoting the UK mpg to an American audience, or the reverse, without saying which gallon.