A report from the Environmental Protection Agency indicates that U.S. automakers' progress toward better fuel economy and emissions standards is slowing.

The agency's new study shows scant improvement in either area. Average fuel economy for U.S. passenger vehicles in 2014, industrywide, was 24.3 miles per gallon - the same number achieved in 2013. The average of carbon dioxide emissions was also the same, at 366 grams per mile.

This represents backsliding on the part of automakers and auto consumers, critics were quick to say.

Automakers sold more vehicles that were heavier, larger, and produced more horsepower, according to the EPA numbers. Consumers bought more light trucks and SUVs, and fewer passenger sedans.

The numbers end a years-long trend of improving progress toward the government's stated goal of an average 54.5 miles per gallon by 2025. The Obama administration said in 2012 that automakers would have to hit that goal or face penalties.

Fuel economy rose in an almost unbroken line from 2004 to 2013, with one small blip downward in 2011, from an average in 2004 of 19.3 miles per gallon to the 2013 average of 24.3.

Dan Becker, director of Safe Climate Campaign, said the report demonstrates that car companies are using government loopholes that allow weaker fuel and emissions standards for SUVs, trucks and larger vehicles, and called upon them to behave more responsibly.

"The auto industry is exploiting the program's loopholes to boost gas-guzzler production and thwart the rules," Becker said. "Automakers should deliver cars and trucks that meet the standards without relying on loopholes to comply with the law."

The EPA report shows that the 15.5 million passenger vehicles sold in 2014 were, on average, 57 pounds heavier, had a wider footprint, and produced four more horsepower than their 2013 counterparts.

That's in part because more of them were SUVs and trucks. About 40 percent of new vehicles sold to American consumers in 2014 fell into that category - up from 35 percent in 2013 and only 19 percent when the EPA began tabulating such numbers in 1975.

"The biggest factor is that the carmakers have been producing more trucks than cars and larger cars," Becker said.

Another factor: lower gas prices, which analysts have said encourage consumers to favor heavier, faster vehicles that use more fuel.

At this time last year, the EPA's report on 2013 fuel economy and emissions represented all-time records, with numbers that had improved in eight of nine preceding years.

The model year 2013 fuel economy was an industry average of 24.1 miles per gallon, a 0.5-mile-per-gallon improvement from 2012. The 2013 number represented a 20 percent increase in fuel economy above 2004 averages.

Over the same period, average carbon dioxide emissions for light duty gasoline and diesel vehicles was 369 grams per mile, a 7 grams per mile improvement over 2012, last year's EPA report said.

On a brighter note, the EPA's preliminary numbers for 2015 show some improvements. Average carbon dioxide emissions should fall to 360 grams per mile, the study said, while average fuel economy should rise to 24.7 miles per gallon.