1. Place the value in liters/100 kilometer (represented by l/100 km in the formula) that you want to convert to miles/gallon to replace the blank space below. This is based on the fact that 1l/100km = 235.214mpg (that means that if 1 liter could go 100 km then 1 gallon would go 235.214 miles).d. 2. Perform the above calculations to get the end result in miles/gallon or mpg. Note the 1/(x g/100km) represents the reciprocal of x g/100, which was found by the preceding two calculation and then inverting that with 1/x which represents a common button on calculators for reciprocal, l/100 km and miles.