DETROIT — U.S. vehicles are losing weight, and it’s helping them use less fuel.

The U.S. Environmental Protection Agency said Wednesday that the average fuel economy of 2015 model-year vehicles increased 0.5 mile per gallon to a record high of 24.8 miles per gallon.

Mazda had the highest average fuel economy of 29.6 mpg, the EPA said. Fiat Chrysler had the lowest, at 20.8 mpg. Most manufacturers improved over 2014, but General Motors Co. and Toyota Motor Corp. both had lower fuel economy in 2015 because they produced more trucks.

Weight loss is one reason that fuel economy is increasing. Vehicles were an average of 25 pounds lighter in the 2015 model year than they were the year before, because automakers are increasingly using lighter materials like aluminum and high-strength steel. Vehicle weight is expected to drop another 50 pounds in 2016, the agency said.

Trucks saw the biggest weight declines in 2015, losing an average of 110 pounds, the EPA said. Ford Motor Co. released its aluminum-bodied F-150 pickup truck in the 2015 model year; it gets up to 22 mpg in city and highway driving. Car weights remained flat from the prior year.

Christopher Grundler, the Director of the Office of Transportation and Air Quality, said new engine and transmission technology also is making vehicles more efficient. Engines with gasoline direct injection — which waste less fuel than traditional engines — are expected to be used in half of new vehicles in the 2016 model year, up from just 3 percent in 2008.

“The main conclusion of looking at this data is that the entire American fleet is getting cleaner every year,” Grundler told The Associated Press. “Every part of that fleet, from pickup trucks to small sedans, is getting better.”

But Daniel Becker of the Safe Climate Campaign, an environmental advocacy group, said the industry isn’t improving as quickly as it should, in part because it keeps selling large SUVs.