To convert from a metric system fuel efficiency measurement like 5 liters per 100 kilometers into the equivalent miles per gallon rating, convert from 100/5 km/liter to mpg.  To convert from an English system measurement like 25 miles per gallon into the equivalent liters per 100 kilometers rating, convert from 100/25 km/mpg to liters; the result will be the number of liters per 100 kilometers.