People in the market for a new vehicle often consider fuel economy to be an important factor in their decision, and automakers have been touting improved gas mileage in new models. However, researchers at the University of Michigan Transportation Research Institute say the average fuel economy of all vehicles on the road has improved by less than one mile per gallon in the past 25 years and less than four miles per gallon since 1923.

Michael Sivak and Brandon Schoettle say the actual on-road fuel economy for all vehicles in the United States—including trucks, buses, and motorcycles—was 17.6 miles per gallon in 2013. This marked only a modest improvement from 16.9 miles per gallon in 1991 and 14 miles per gallon in 1923.

The researchers say a slow turnover in the overall vehicle fleet results in only gradual changes to the average fuel economy. Though newer and more fuel-efficient vehicles help improve the overall fuel economy when they enter the fleet, the average is lowered by older gas guzzlers.

The UMTRI study updates research by Sivak and Schoettle conducted six years ago. Changes in fuel economy in the overall fleet and among different vehicle classes were calculated using information on distances driven and fuel consumed.

Efficiency in the overall vehicle fleet actually declined between 1923 and 1973, falling from 14 miles per gallon to 11.9 miles per gallon. Improvements in fuel economy were spurred by OPEC's decision to reduce oil production and ban exports to the United States in 1973, which led to gas shortages as well as higher prices at the pump.

"After the 1973 oil embargo, vehicle manufacturers achieved major improvements in the on-road fuel economy of vehicles," said Sivak. "However, the slope of the improvement has decreased substantially since 1991."

Fuel economy in light vehicles—including cars, pickup trucks, SUVs, and vans—improved 52 percent between 1973 and 1991. However, it only increased by 10 percent, from 19.6 miles per gallon to 21.6 miles per gallon, between 1973 and 1991.

Sivak and Schoettle say fuel economy among cars averaged 13.4 miles per gallon in 1973, then improved to 21.2 miles per gallon in 1991 and 23.4 miles per gallon in 2013. Light trucks—including pickups, SUVs, and vans—jumped from 9.7 miles per gallon in 1973 to 17 miles per gallon in 1991, but only improved another 0.2 miles per gallon by 2013. Between 1966 and 2013, medium- and heavy-duty trucks saw their fuel economy average increase less than one mile per gallon, from 5.6 miles per gallon to 6.4 miles per gallon.

The researchers conclude that efforts to improve fuel economy in the vehicle fleet should focus on the least efficient vehicles within each class. They note that vehicle classes with the lowest fuel economy have other relevant measures as well, such as ton miles per gallon for medium- and heavy-duty trucks and passenger miles per gallon for buses.

The study gives an example of a one mile per gallon improvement in two different vehicles to illustrate how a focus on the least efficient vehicles can have a significant effect. The improvement only saves seven gallons of gasoline per year in a vehicle getting 40 miles per gallon and traveling 12,000 miles a year, but the savings jump to 50 gallons a year for a vehicle with 15 miles per gallon traveling the same distance.

A new federal benchmark set in 2012 requires new vehicles in the U.S. fleet to average 54.5 miles per gallon under the Corporate Average Fuel Economy standards by 2025. This measure is different from the window sticker values seen at a dealership and set by the Environmental Protection Agency, so vehicles meeting the CAFÉ standard would have a window sticker value of about 36 miles per gallon.

Even with more efficient automobiles entering the U.S. fleet with each new vehicle purchase, improving the fleet's average fuel economy might be a slow process. A recent analysis by IHS Automotive found that the average age of an American light vehicle at the start of 2015 was 11.5 years, with new vehicles making up only 6.5 percent of the 252.7 million vehicles currently on the road.

Sivak and Schoettle have also tracked the average fuel economy in new light vehicle sales since October of 2007. This figure has climbed from 20.1 miles per gallon in that month to a peak of 25.8 miles per gallon in August of 2014. The monthly average fuel economy in new vehicles has trended lower, varying between 25 miles per gallon and 25.5 miles per gallon. The researchers attribute the lower averages to the continued trend of lower gas prices than the previous year.