The Trump administration is expected to announce soon that it will roll back automobile gas mileage and pollution standards that were a pillar in the Obama administration's plans to combat climate change.

It's not clear whether the announcement will include a specific number, but Obama-era rules require the fleet of new vehicles to get 54.5 miles per U.S. gallon (4.3 litres/100 km) by 2025. The Environmental Protection Agency (EPA), applying a new standard based on real world driving, says that works out to 36 miles per gallon (7.8 litres/100 km).

Environmental groups, who predict increased greenhouse gas emissions and more gasoline consumption if the standards are relaxed, say the announcement could come Tuesday at a Virginia car dealership. An EPA spokesperson did not immediately respond to requests for comment.

Conflict with California rules

Any change is likely to set up a legal showdown with California, which currently has the power to set its own pollution and gas mileage standards and doesn't want them to change. About a dozen other states follow California's rules, and together they account for more than one-third of the cars sold in the U.S.

Automakers have lobbied to revisit the requirements, saying they'll have trouble reaching them because people are buying bigger vehicles due to low gas prices. They say the standards will cost the industry billions of dollars and raise vehicle prices due to the cost of developing technology needed to raise mileage.

But environmental groups say people will save money at the pump, and the technology is available for the industry to comply.

They also say burning more gasoline will put people's health at risk.

"The American public overwhelmingly supports strong vehicle standards because they cut the cost of driving, reduce air pollution and combat climate change. Backing off now is irresponsible and unwarranted," said Luke Tonachel, director of the Natural Resources Defence Council's Clean Vehicles and Fuels Project.

The EPA and the National Highway Traffic Safety Administration are involved in setting the standards, which would cover the years 2022 through 2025.