Fuel efficiency sometimes means the same as thermal efficiency, that is, the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. But 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 kilometres" for an automobile (sometimes called fuel economy). Here, vehicle-miles is the output, but for transportation, output can also be measured in terms of passenger-miles or ton-miles (of freight).