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DETROIT — Since agreeing to tough new federal fuel economy standards five years ago, automakers have been methodically improving the gas mileage of their vehicles and reducing emissions harmful to the environment.
But despite investing billions in fuel-saving technologies and introducing a raft of lower-mileage models and electric cars, the industry will be hard-pressed to meet its target of 54.5 miles per gallon in 2025.
Now, with a crucial midterm review of federal fuel-economy rules to begin this summer, automakers are expected to seek adjustments to the government’s formula for increasing mileage and cutting greenhouse gas emissions.
Proposed changes could include extending the time frame on mileage targets and expanding emissions credits to include enhancements for safety and autonomous driving — such as the move that carmakers announced last week to make automatic braking standard on all models by early next decade.
“We think the government needs to start thinking outside of the box and not just look at the traditional ways of reducing greenhouse gases,” said Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers in Washington.
An industry effort to relax the mileage goals could set off conflict with the Obama administration, whose pledge to reduce emissions, as part of last year’s Paris climate accord, includes making big gains in vehicle fuel efficiency.
For now, government officials and auto executives are taking a cooperative stance and extolling the gains in fuel economy in recent years rather than questioning the industry’s ability to reach much higher standards down the road.
In a progress report released in December, the Environmental Protection Agency said fuel economy for new vehicles sold in the United States had improved 26 percent from 2004 to 2014, to 24.3 miles per gallon.
“It’s clear that our standards are working,” said Christopher Grundler, director of the agency’s Office of Transportation and Air Quality.
The 24.3 figure is based on real-world driving performance, and it is lower than the mileage numbers used to calculate the government’s Corporate Average Fuel Economy. For CAFE purposes, the average for 2014 was about 31 miles per gallon.
Individual carmakers have also been able to increase their CAFE scores by using a complex menu of emissions credits issued by the government for technical innovations like low-energy headlights and advanced air-conditioning systems.
Over all, most major automakers are on track to achieve CAFE’s unadjusted midterm target of 37 miles per gallon next year. Some of them, like Fiat Chrysler Automobiles, are on pace only because they have bought emissions credits from other companies.
But improving fuel economy significantly across vehicle fleets is becoming more challenging and expensive.
“Now they are complying, but they are worried that in the future, the climb gets much steeper,” Ms. Bergquist said.
Cheap gas prices are prompting more consumers to buy trucks and sport utility vehicles instead of small cars, hybrids or pure electric vehicles.