Define fuel efficiency.

Fuel efficiency, often referred to as "fuel economy," is a measure of how efficiently a vehicle or machine converts the energy stored in fuel into useful work, typically measured in terms of distance traveled per unit of fuel consumed. It is commonly expressed as miles per gallon (mpg) or kilometers per liter (km/l), depending on regional preferences.

In the context of vehicles, higher fuel efficiency indicates that the vehicle can travel a greater distance using less fuel. This is desirable for several reasons, including reducing operating costs for the vehicle owner, decreasing the environmental impact by reducing greenhouse gas emissions and dependence on fossil fuels, and increasing energy sustainability.

Fuel efficiency can vary significantly between different types of vehicles, such as cars, trucks, and motorcycles, as well as between various modes of transportation, such as gasoline-powered, diesel-powered, electric, or hybrid vehicles. It can also be influenced by factors like driving habits, vehicle maintenance, and technological advancements in engine design and fuel consumption management.