Fuel economy is the amount of fuel required to move a vehicle over a given distance. Fuel efficiency is the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. Fuel efficiency can also mean the output one gets for a unit amount of fuel input such as "miles per gallon" or "liters per 100 kilometers" for an automobile (sometimes called fuel economy).  Automobile fuel efficiency can be calculated in either l/100 km or MPG  (Litres per 100 km).  This Online Fuel Efficiency Calculator can calculate l/100 km.