The average efficiency increase to 24.9 miles per gallon was modest, up 0.2 mpg from 24.7 miles per gallon in 2016, its growth stunted in part because Americans are buying more large SUVs and pickups and fewer cars.
EPA Administrator Andrew Wheeler said "there are legitimate concerns about the ability to cost-effectively achieve the Obama administration's standards in the near future" and he noted that just three out of 13 major automakers met the 2017 requirements without using credits.
Automakers earn credits for overcomplying with fuel efficiency requirements and can save them for use in future years when they face tougher standards or sell them to competitors if they do not need them. Automakers worry that without significant changes, they will not be able to meet requirements after 2020 because many credits expire.
The EPA report also shows that preliminary fuel efficiency for 2018 jumped by 0.5 mpg to 25.4 mpg.
Luke Tonachel, director for clean vehicles and fuels at the Natural Resources Defense Council, said the "data shows the current standards are working" and suggests that automakers could adopt additional existing technologies to improve fuel efficiency.
The report shows that Fiat Chrysler Automobiles NV bought a significant number of credits, while Honda, Nissan and Tesla were among those selling credits. BMW AG, Jaguar Land Rover, Volkswagen AG, and Daimler AG also bought credits, the report said.
Fiat Chrysler disclosed last month it paid $77 million in U.S. civil penalties for failing to meet 2016 model year fuel economy requirements.
In August, the EPA and National Highway Traffic Safety Administration unveiled a proposal to freeze fuel economy requirements at 2020 levels through 2026, a move it said would save automakers more than $300 billion in regulatory costs and reduce traffic deaths.
The Obama administration adopted rules calling for a nearly 5 percent annual increase in fuel efficiency requirements from 2021 through 2026.