The average fuel economy of the new cars and light trucks sold in the U.S. is appreciably higher than it was 10 years ago.

The University of Michigan Transportation Research Institute has demonstrated that in its regular reports calculating new-car average fuel economy since October 2007.

But researchers Michael Sivak and Brandon Schoettle found progress to be less clearcut when they looked at the aggregate fuel economy of all cars on U.S. roads—a fleet that now numbers roughly 250 million vehicles.

DON'T MISS: Calculated average new-car fuel economy in 3-year plateau

While the fuel economy of new vehicles sold has improved for the most part, the gas mileage of the overall U.S. vehicle fleet has only increased slightly over the last 25 years, researchers found.

A report on the fuel economy of vehicles on U.S. roads between 1923 and 2015 found that fuel economy fluctuated for most of the 20th century, with smaller improvements over the last 25 years.

Overall fleet fuel economy in 2015 averaged 17.9 mpg, compared to 16.9 mpg in 1991.

1974 Ford Mustang II

1974 Ford Mustang II

And the year 1991, in fact, marked the end of a period of major fuel-economy improvements.

Overall fleet fuel economy stood at 11.9 mpg in 1973, which was actually a decrease from the 14.0 mpg recorded half a century earlier in 1923, according to the report.

The steep increase in fuel economy between 1974 and 1991 resulted from the 1973 oil crisis and ensuing legislation in 1975 to raise fuel efficiency for both national security and environmental reasons.

ALSO SEE: Overall U.S. Fuel Economy: Higher Now Than In 1923, But Only A Little (Aug 2015)

Researchers calculated the fleet fuel-economy averages by analyzing distances driven and fuel consumed, with averages calculated for different classes of vehicle spanning most light-duty categories: cars, pickup trucks, SUVs, and vans.

Even large increases in the fuel efficiency of new vehicles take time to make a substantial impact on the overall vehicle fleet.

That's because new cars must displace older, less efficient models to register a true reduction in fuel consumption.

Automobile accident, Washington, D.C., 1923

Automobile accident, Washington, D.C., 1923

The average age of a car on U.S. roads is close to 12 years old—the highest since World War II.

Both the increasing reliability of modern cars and the lingering effects of the Great Recession have influenced Americans to keep their cars longer.

At the same time, the rate of fuel-efficiency improvements for new cars appear to be slowing down.

MORE: Modern electric cars at 20: from EV1 to Bolt EV, where are we now? (Dec 2016)

New-car average fuel economy stayed at the same level—about 25 mpg—between 2014 and 2016, according to UMTRI.

This stagnation roughly corresponds with a period of low gas prices, which are widely blamed with driving consumers toward less-efficient vehicles.

Sedans and hatchbacks made up just 37 percent of new-vehicle sales last month, a record low. The balance, 63 percent, was composed of light trucks: SUVs, crossover utility vehicles, minivans, and pickup trucks.


Follow GreenCarReports on Facebook and Twitter