by Michael Sivak.

This is the latest of my occasional updates on retrospective, long-term trends in U.S. vehicle fuel economy. Specifically, this study examines actual fuel economy of cars and light trucks (pickup trucks, SUVs, and vans) from 1966 through 2017.

Calculated vehicle fuel economy is available going back to 1923. However, the starting year of this analysis is 1966 because (1) for 1923 through 1935, fuel-economy information is available only for the entire fleet of all vehicles, and (2) from 1936 through 1965, light trucks were included with other trucks in the relevant statistics. 

The primary measure in this analysis is the average miles driven per gallon of fuel for each year. For 1966 through 1985, fuel economy was calculated from the information in a summary publication by the U.S. Department of Transportation. Annual statistics from the U.S. Department of Transportation were the sources of the information for 1986 through 2017.

The graph below shows the changes in actual vehicle fuel economy from 1966 through 2017.

Noteworthy fuel-economy trends, taking into account the length of time represented:

• A minor decrease between 1966 and 1973 (from 13.5 mpg to 12.9 mpg).

• A modest increase between 1973 and 1991 (from 12.9 mpg to 19.6 mpg).

• No change between 1991 and 2004 (19.6 mpg for both years).

• A modest increase between 2004 and 2008 (from 19.6 mpg to 21.8 mpg).

• A minor increase between 2008 and 2017 (from 21.8 mpg to 22.3 mpg).

In terms of fuel consumption per distance driven, the change between 1973 (the year of the first oil embargo) and 1991 (from 7.75 gallons per 100 miles to 5.09 gallons per 100 miles) represents a compound rate of decrease of 2.3% per year. In comparison, the change between 2008 and 2017 (from 4.59 gallons per 100 miles to 4.48 gallons per 100 miles) represents a compound rate of decrease of only 0.3% per year.

One fundamental hurdle to improving the average fuel economy of the entire fleet is that improvements in fuel economy for new vehicles take a long time to substantially influence the fuel economy of the entire fleet. This is the case because it takes many years to turn over the fleet. For example, the 17.2 million cars and light trucks sold in the United States in 2017 accounted for only 6.9% of the entire fleet of 250.6 million registered cars and light trucks, and the average age of cars and light trucks on the road in 2017 was 11.7 years.

The measures used in the present analysis (miles per gallon of fuel, and gallons of fuel per 100 miles) do not allow us to make direct inferences regarding changes over time in total energy consumed (e.g., total Btu). This is the case because the analysis did not take into account the following factors and their changed contributions over time: greater energy content of diesel than gasoline, lower energy content of ethanol blends than pure gasoline, lower energy content of biodiesel blends than pure diesel, and electric energy consumed by plug-in electric vehicles. (Concerning the latter: Even if all 755,000 plug-in cars and light trucks that were sold in the U.S. through December 2017 were still on the road, they would represent only about 0.3% of all registered cars and light trucks in 2017.)

Michael Sivak is the managing director of Sivak Applied Research and the former director of Sustainable Worldwide Transportation at the University of Michigan.