The U.S. Environmental Protection Agency said the fuel economy for new vehicles hit a new high in the 2020 model year at 25.4 miles per gallon but most automakers relied on credits to meet federal emissions requirements.
The agency expects the industry's fuel economy for the 2021 model year will decline to 25.3 mpg.
The Detroit 3 automakers -- General Motors, Ford Motor Co. and Chrysler parent Stellantis -- lagged other automakers in fuel economy performance, according to the EPA report, as they sell a rising number of large trucks and SUVs.
GM is projected to be the lowest of all automakers in 2021 with a fleet average of 21.5 mpg for all its vehicles, just behind Stellantis at 21.6 mpg, with Ford at 22.7 mpg.
The report said since 2004, carbon dioxide emissions have decreased by 24 percent as fuel economy has improved by 32 percent.
The report said only Tesla Inc., Subaru Inc. and Honda Motor Co. met requirements without using credits. Automakers can either purchase credits or use credits earned from earlier model years.
The report shows GM, Daimler AG's Mercedes and Stellantis all bought significant credits in 2020, while Tesla, Honda and Toyota Motor Corp. all sold credits.
In August, the EPA proposed reversing the Trump-era loosening of vehicle emissions rules with a new plan to boost efficiency 10 percent in the 2023 model year, aiming for a fleet average of 52 mpg by 2026.
The American Council for an Energy-Efficient Economy said "automakers are producing cars that are barely more fuel efficient on average than what they sold a year ago."
Dan Becker, director of the Safe Climate Transport Campaign, said the report shows "auto companies have made sky-high promises to turn out clean cars, but the EPA’s report shows they have produced very few."
He said the 2020 improvement -- just 1.9 percent over 2019 -- showed that "with weak rules, automakers will keep pushing gas guzzlers, sticking consumers with high gas bills and plentiful pollution."
For 2020, GM and Ford both averaged 23 miles per gallon, while Stellantis averaged 21.3 mpg. Tesla, which makes only electric vehicles, led the industry, while Honda averaged 29.1 mpg, which was first among automakers building gas-powered vehicles.
The report showed that over the last five years, GM's and Ford's performances barely improved, while Stellantis' fell by 0.5 mpg.
Stellantis said on Friday it is investing more than $35 billion through 2025 to develop battery electric vehicles but said "consumer and manufacturing incentives and recharging infrastructure" are needed to move the market toward EVs.
The report also showed average vehicle weight in 2020 rose to another all-time high of 4,166 pounds, while average horsepower and size both also set records.