The fuel economy of U.S. vehicles has increased by 2.4 mpg since 2011, the year before Greenhouse Gas and Fuel economy standards for passenger vehicles took effect, while emissions per mile traveled decreased by 10 percent, the EPA said.
But automakers' margin of compliance over the requirements has fallen as the rules have gotten tougher, the agency said.
Grundler told Reuters the report showed the fuel rules were working and automakers are "on track" to meet future requirements.
Low oil prices are prompting a shift away from cars toward crossovers, SUVs and pickup trucks and major automakers and some in Congress have raised concerns that the final years of fuel rules, which are tougher on larger vehicles than in earlier years, may not be achievable.
U.S. regulators must decide by April 2018 whether the 2022-25 standards are appropriate or should be changed.
The report said "the market shift towards SUVs has offset some of the fleetwide benefits that otherwise would have been achieved."
The EPA said the average fuel economy for cars rose to 28.6 mpg, a 0.7 mpg increase over 2014, while 2015 light trucks increased 0.7 mpg to 21.1 mpg.
The report said Mazda Motor Corp. had the highest overall fuel efficiency, followed closely by Honda Motor Co., Nissan Motor Co., Subaru and Hyundai Motor Co.
Automakers that do not sell pickups have higher fuel efficiency than other carmakers.
Fiat Chrysler Automobiles NV had the least fuel-efficient fleet, but also had the biggest one-year improvement.
Fiat Chrysler purchased 1.34 million megagrams of emissions credits from Tesla Motors Inc. in the 2015 model year, the report shows. Fiat Chrysler has purchased 15.2 million credits since 2010, mostly from Honda. A megagram is equal to 1,000 kilograms and is calculated on emissions saved over legal requirements.
Toyota Motor Corp.'s overall fuel efficiency fell 0.4 mpg to 25.2 mpg, while General Motors declined 0.5 mpg to 22.8 mpg.
Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers, an industry trade group, said "for now, automakers are ahead in meeting fuel economy targets because they have introduced the affordable fuel-saving technology to consumers as soon as possible. But the challenge of meeting future requirements will require new technology or more costly technology as fuel economy targets become higher."
Dan Becker, director of the Safe Climate Campaign, said automakers are using credits in part to meet the requirements by building vehicles that can run on mostly ethanol or "passive cabin ventilation" aimed at reducing air conditioning. "These are phantom mileage gains," Becker said.
The EPA said improvements such as turbo charging, engine downsizing, improved transmissions, weight reduction and air conditioning system improvements largely account for the gains even as low gas prices shift buying habits.
Reuters contributed to this report.