Ford said in a statement that it is focused on acting to protect the environment while also protecting the affordability of vehicles. "This agreement with California provides regulatory stability while reducing CO2 more than complying with two different standards," it said.
There is no evidence that existing fuel economy rules would degrade vehicle performance. And environmentalists and many states challenge Trump's assertion that his administration's proposed rule would boost vehicle safety or dramatically reduce the price of vehicles -- and argue that consumers will save more in reduced fuel costs under the Obama rules.
California Attorney General Xavier Becerra responded to Trump's attacks on automakers saying it would result in an additional 540 million metric tons of greenhouse gases and other harms. "This doesn't look like a better alternative to us," he said.
The White House has urged other automakers not to back the California agreement, while Democrats have been calling and writing automakers urging them to sign on with California.
The EPA on Tuesday ridiculed the voluntary framework, which it said "so far has been nothing more than a press release."
"My proposal to the politically correct Automobile Companies would lower the average price of a car to consumers by more than $3000, while at the same time making the cars substantially safer. Engines would run smoother. Very little impact on the environment! Foolish executives!" Trump tweeted earlier.
Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers, representing General Motors, Toyota Motor Corp., Ford, Volkswagen and others, said the companies "look forward to seeing a final rule soon. We support increases to standards that optimize all the priorities, including affordability so more Americans can buy a new car, plus preserving jobs and safety at the same time."
GM has not backed the voluntary agreement, arguing that it does not properly credit the company's electric vehicles.
Even so, Trump tweeted that the founders of Ford and GM "are 'rolling over' at the weakness of current car company executives" over the fuel rules, adding: "Crazy!"
GM said late Wednesday that the company is "continuously improving fuel economy" and is focused on "working with all parties on a solution that would involve a 50-state solution and a national electric vehicle program."