The group, Bluewater Network, \"understands the proposed reduction will be 30 percent,\" said Elisa Lynch, the group\'s global warming campaign director.
The organization helped draft the California legislation that led to the nation\'s first law in 2002 to cut vehicle emissions of carbon dioxide and other gases that have been linked to global warming.
The California plan was first reported on Wednesday by the New York Times.
A spokesman for California\'s Air Resources Board said the agency would announce its plan on June 14 but would not comment on its contents until then.
The Air Resources Board is required by state law to produce an emissions reduction plan by 2005 to be reviewed by the state legislature.
The plan would go into effect in 2006 and give car and light truck makers until the 2009 model year to begin meeting emission rules.
The first phase of the plan would require a 23 percent reduction in 2012, and the second phase would mandate the full 30 percent standard due in the 2015 model year, said Bluewater\'s Lynch.
Even before standards for California have been announced, a debate has already begun about whether they go far enough and over how they can be achieved.
The Union of Concerned Scientists, a science and environmental group based in Cambridge, Massachusetts, said that based on its analysis, California could achieve a 40-percent cut in emissions within 10 years.
\"Our view is that 30 percent is a strong but conservative proposal,\" said Jason Mark, clean vehicles director for the scientists\' group. General Motors Corp. spokesman Dave Barthmuss said, \"Our view at General Motors is that innovative technologies like the ones we\'re already putting in our cars and trucks will be the answer. \"Consumers will be able to buy full-sized hybrid pick-up trucks later this summer. We\'re confident hydrogen will be the long-term fuel of choice.... Our goal is to establish commercial viability of hydrogen technology by 2010,\" Barthmuss said.
California drivers can expect to pay \"a few hundred more bucks\" for vehicles with lower emissions, \"but they will make it back in lower operating costs at the pump within three to four years,\" Mark added.
California\'s Air Resources Board \"did a good technical analysis but was too conservative in their economic analysis to get a higher reduction,\" said Lynch.
A spokesman for Gov. Arnold Schwarzenegger was not available to comment on the emissions plan.
California also is looking at reducing emissions from heavy-duty diesel trucks, buses and recreational vehicles built between 1993 and 1999.