The fuel economy of an automobile relates distance traveled by a vehicle and the amount of fuel consumed. Consumption can be expressed in terms of volume of fuel to travel a distance, or the distance traveled per unit volume of fuel consumed. Since fuel consumption of vehicles is a significant factor in air pollution, and since importation of motor fuel can be a large part of a nation's foreign trade, many countries impose requirements for fuel economy. Different methods are used to approximate the actual performance of the vehicle. The energy in fuel is required to overcome various losses (wind resistance, tire drag, and others) encountered while propelling the vehicle, and in providing power to vehicle systems such as ignition or air conditioning. Various strategies can be employed to reduce losses at each of the conversions between the chemical energy in the fuel and the kinetic energy of the vehicle. Driver behavior can affect fuel economy; maneuvers such as sudden acceleration and heavy braking waste energy. Electric cars do not directly burn fuel, and so do not have fuel economy per se, but equivalence measures, such as miles per gallon gasoline equivalent have been created to attempt to compare them. Units of measure Fuel economy is the relationship between the distance traveled and fuel consumed. Fuel economy can be expressed in two ways: Units of fuel per fixed distanceGenerally expressed in liters per 100 kilometers (L/100 km), used in most European countries, China, South Africa, Australia and New Zealand. Irish law allows for the use of miles per imperial gallon, alongside liters per 100 kilometers. Canadian law requires fuel economy to be measured in both liters per 100 kilometers and miles per imperial gallon. Liters per 100 kilometers may be used alongside miles per imperial gallon in the UK. The window sticker on new US cars displays the vehicle's fuel consumption in US gallons per 100 miles, in addition to the traditional mpg number. A lower number means more efficient, while a higher number means less efficient. Units of distance per fixed fuel unit Miles per gallon (mpg) are commonly used in the United States, the United Kingdom, and Canada (alongside L/100 km). Kilometers per liter (km/L) are more commonly used elsewhere in the Americas, Asia, parts of Africa and Oceania. In the Levant km/20 L is used, known as kilometers per tanaka, a metal container which has a volume