The official definition of fuel economy is how much gas is needed to move a certain distance. In the United States, fuel economy is measured in miles per gallon, while in the rest of the world, it is mostly calculated in liters per one hundred kilometers. These measuring systems vary greatly; a higher number in the US system is optimal, while a lower number is desired in the metric system (the lower the number, the lower the amount of fuel needed to travel one hundred kilometers).

Governments are taking notice of the importance of fuel economy, and are establishing guidelines which will regulate efficiency in the future. The European Union has decided that, starting in 2012, gasoline cars must have at least a 47mpg rating, and diesel cars must have at least a 52mpg rating. The United States currently taxes vehicles that do not adhere to environmental standards, as a means of discouraging their production. These taxes and penalties are expected to increase drastically within the next few years.

Engineers are constantly working to find ways to increase the fuel efficiency of vehicles. Several factors affect fuel economy, so it is a matter of altering those factors to produce better efficiency. The weight of a car is one of the biggest factors in fuel economy, so engineers are always trying to make cars lighter.

This can be done through replacing traditional materials, such as iron, with alternative options like carbon fiber. Wind resistance and drag require cars to exert energy to overcome them, so vehicles that are designed to experience these factors minimally have better fuel economies. Anything that can increase or save energy is beneficial. For example, shutting off the engine while the car is stopped saves energy.

Improving the efficiency of engine combustion is a key factor. Most engines only exploit 10-20% of the energy released from burning fuel; the rest is lost to heat and other forms of energy. Fuel cells are so desirable because up to 80% of their produced energy can be directly utilized. Different types of combustion, such as lean burn and HCCI combustion are being tested to identify what method is most efficient.

Average consumers can play their part in increasing the fuel economy of their vehicles. Fuel efficiency increasing products are popular in the market right now, but most of them do not actually improve efficiency at all. Simple things, like keeping the air pressure in tires high, and slowly accelerating and decelerating, can expand a vehicle's rating a fair degree.

Hybrid cars are the best option currently for those who want speed and great fuel economy. These models combine electrical and gasoline engine components to create something which does not feel like an electric car at all. Hybrid vehicles recapture wasted energy during breaking, shut off gasoline power during idle periods, and utilize gas and electric power simultaneously to reduce the total amount of fuel consumed. All of these factors combined result in fuel ratings of over 50mpg, as well as lower emissions.