Detroit and Japanese automakers have closely tracked the proposal because California accounts for nearly 13 percent of the U.S. auto market and other states may adopt similar rules.

Automakers have said in the past that they may sue to block the plan, but a legal challenge so far has not surfaced.

"We're looking at our options," said Gloria Bergquist, spokeswoman for the Alliance of Automobile Manufacturers, a Washington, D.C.-based group that represents Ford Motor Co., General Motors Corp., DaimlerChrysler and six other automakers.

In its initial phase from 2009 through 2012, the plan calls for regulations requiring technologies to reduce emissions by about 25 percent for cars and light trucks and by about 18 percent for larger trucks and sport-utility vehicles.

When fully implemented after 2016, the recommended regulations would slash emissions by up to 34 percent for cars and light trucks and by 25 percent for larger vehicles.

In the initial phase, the new rules would add about $292 to the cost of each car and small truck and about $308 to the cost of a large pickup and SUV. In the next phase, between 2013 and 2016, it would add an average of $626 per car and $955 per large pickup and SUV, according to the board.

Bergquist said she expects the air board to clear the plan and California drivers will get "no apparent health or environmental benefits" from it. She estimated it will cost drivers $6 billion a year for added vehicle costs.

The next stop for the emissions plan will be the California Legislature, which has until the end of 2005 to review it, said Jerry Martin, a spokesman for the air board.

"I would think we're looking at a suit next year after we see what the Legislature does. The federal government is the only one who has the authority to regulate fuel economy standards," said a source familiar with the auto industry's plans.

The Bluewater Network, an environmental group in San Francisco, said the emissions plan is "a good start" but could have gone further.

"It won't come anywhere near what we need to do to curb global warming. Cars will be cleaner, but there will be more on the road driving more miles. That means higher emissions," said Elisa Lynch, the group's director for a global warming campaign.

(Additional reporting by Michael Ellis in Detroit)