“Energy efficiency” in transportation is generally discussed using terminology specific to this sector of the economy, as defined below. The primary terms used to quantify the fuel consumed by a vehicle as it is driven are “fuel economy” (or “fuel efficiency”) and “fuel consumption.”

Fuel efficiency is a relative term used to describe how effectively fuel is used to move a vehicle. Thus, a heavy and a light vehicle, using the same technology in the same ways, would have the same fuel efficiency but very different fuel economy. Note that fuel-efficiency improvements do not necessarily result in increased fuel economy, as they are often offset by the negative effects of increases in vehicle power and weight. Thus, fuel efficiency is related to the amount of useful work that is derived from the combustion of fuel. Whether that useful work is applied to increase the number of miles that can be traveled per gallon of fuel or to provide other amenities (such as size and power) is a separate question.

Fuel economy is expressed as miles per gallon of fuel consumed; it is the term most commonly used in the United States in discussing vehicle fuel consumption.

Fuel consumption is the inverse of fuel economy. It refers to the fuel consumed by the vehicle as it travels a given distance. Widely used in the Europe (expressed in liters per 100 km), this metric is a clearer measure of fuel use than is fuel economy. The amount of fuel consumed in driving from one place to another (say, New York City to Washington, D.C.) is what matters to consumers. In U.S. units, fuel consumption is usually expressed as gallons per 100 miles.

To illustrate: A vehicle with fuel economy of 50 miles per gallon (mpg), which corresponds to fuel consumption of 2 gallons per 100 miles, is twice as fuelefficient as a vehicle of the same size, weight, and power that gets 25 mpg, corresponding to 4 gallons per 100 miles.