Automobile Energy Use, Efficiency and Emissions Explained
The further a vehicle travels on a given amount of fuel, the more efficiently it has performed. Fuel efficiency is measured differently in Canada and the U.S. In the U.S., it is measured as fuel economy or mileage (miles driven per gallon consumed); in Canada, it is measured as fuel consumption (litres consumed per hundred kilometers). This can be confusing since as fuel efficiency improves (i.e. you can travel further on the same amount of fuel) fuel economy increases, but fuel consumption decreases.
Of the energy supplied to an automobile by the fuel, only about 13 per cent makes it to the wheels under city driving conditions and 20 per cent under highway conditions. Most of the potential energy is lost through internal energy losses. In particular, friction within the engine and drivetrain (i.e., the series of components that link the engine and the wheels) causes almost 63 per cent of energy to be lost as heat.
Further energy is lost due to external energy loads: wind resistance (aerodynamic drag), friction between the tires and the road (rolling resistance), and inertia (the energy needed to accelerate the car). The effect of these external energy loads is directly associated with vehicle shape, vehicle weight and tire quality. Changes in engineering that can be made to lessen these impacts are discussed further on the page Increasing Fuel Efficiency by Improving Automobile Technology. For more information on how driving practices can help to lower these effects, please see the What You Can Do.
Everything you ever wanted to know about fuel efficiency appears in our Primer on Automobile Fuel Efficiency and Emissions. Take a look for yourself! 