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Not all of the fuel that is put into a car's fuel tank is used to move the car down the road. In fact, only 14-16% of the energy is used for that purpose. The rest of the energy is lost to engine inefficiencies or used to power accessories. The amount of energy losses is different depending on the type of driving โ€“ city, highway, or combined city and highway. The engine losses, such as exhaust heat and pumping, are higher for city driving than for highway driving. There are no idle losses in highway driving, but losses due to wind resistance and rolling resistance are higher for highway driving than city driving. All in all, there is great potential to improve vehicle fuel efficiencies with advanced technologies that address these losses.

Graphic of an automobile showing energy requirements for combined city/highway driving (drivetrain losses, engine losses, parasitic losses, and power to wheels. For more detailed information, see supporting information below.

Share of Energy Losses in a Typical Car
Graphic showing share of energy losses in a typical car. For more detailed information, see supporting information below.

Supporting Information

Share of Energy Losses in a Typical Car
Loss Type Combined City Highway
Engine Losses 70-72% 74-75% 65-69%
Thermal - radiator, exhaust heat, etc. 60-62% 63-64% 56-60%
Combustion 3% 3% 3%
Pumping 4% 5% 3%
Friction 3% 3% 3%
Parasitic Losses, e.g. water pump, alternator, etc. 5-6% 6-7% 3-4%
Power to Wheels, dissipated as: 17-21% 14-16% 20-26%
Wind Resistance 8-10% 8-10% 13-16%
Rolling Resistance 5-6% 4% 5-6%
Braking 4-5% 4-5% 4-5%
Drivetrain Losses 5-6% 6-7% 5-6%
Idle Losses 3% 6% 0%

Source: U.S. Department of Energy and U.S. Environmental Protection Agency, Fuel Economy Guide website.

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