How this calculator works
Fuel mileage is distance per unit of fuel, while L/100 km is fuel per unit of distance: one is the inverse of the other after unit conversion. Distance and fuel must come from the same interval, most reliably between two full-tank refills. The US gallon and the Imperial gallon are different sizes, so the two mpg figures differ even though the underlying measurement is identical.
Formula
mpg_us = (distance_km / 1.609344) / (fuel_litres / 3.785411784). mpg_imperial = (distance_km / 1.609344) / (fuel_litres / 4.54609). litres_per_100km = 100 x fuel_litres / distance_km.
Worked example
A 300-mile trip on 10 US gallons, entered as 482.8032 km and 37.85411784 litres, gives 30 US mpg and about 36.03 Imperial mpg, or about 7.84 L/100 km.
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Frequently asked questions
Is a higher mpg better?
Yes, for mpg a higher number means less fuel per distance. For litres per 100 km the direction reverses: a lower number is better.
Which gallon does a Canadian mpg figure use?
It depends on the source. Older Canadian material often used the Imperial gallon, while modern comparisons and US sources use the US gallon, which is smaller, so the same car shows a lower US mpg number.
Why does my trip computer differ from this result?
A trip computer estimates from injector data and may reset between fills. The full-tank method measures the fuel you actually bought over the distance you actually drove, so small differences are normal.