Fuel economy is the measure of how much fuel is required to move a vehicle a certain distance. It is typically expressed in miles per gallon (MPG) in the U.S., and in an inverse form, liters per 100 kilometers traveled in Europe and elsewhere. (The Japanese split the difference, using km/liter.) As more interest is paid around the world to fuel economy (in connection with dwindling oil reserves), at least two problems with these measures arise. First, none of them really relates to the primary goal of driving a car, which is to move one or more passengers. When there is just one person in the car, most of the fuel is burned up in moving empty seats; thus the "true" fuel economy of a 20 MPG car loaded with four people may be greater than that of a 40 MPG car with only the driver aboard. Secondly, these measures were derived when our only fuel choices were gasoline or diesel. As fuels such as electricity and hydrogen move into the picture, there is increasing need for an "MPG equivalency" measure, which can put all vehicles on a common footing (a battery-powered vehicle, for example, notionally but nonsensically gets an infinite number of miles per gallon).