Adapting to the new rules would take far longer, said Thomas Austin, an industry expert at Sierra Research Inc., in court testimony.

"We still need 12 years from now, we're talking model year 2020," to develop full lines of compliant vehicles, he said.

Vermont is one of 10 US states to call for the sharp reduction in emissions of CO2, a greenhouse gas that most scientists say contributes to global climate change.

The New England state, known for its rural beauty, argued that it needs to act on CO2 emissions to protect its skiing, tourism and agriculture industries, which are dependent on the state's cold, snowy winters and temperate summers.

The Union of Concerned Scientists, not a party to the case, said that by making the most of emissions-reducing technologies already in use it has developed models for a variety of vehicles, including a six-seater minivan called the Vanguard, that have up to 40 percent lower greenhouse gas emissions -- though not all of that comes from reduced carbon dioxide emissions.

"This is possible, and this ends up saving the consumer money in the long run because you reduce the operating costs of the vehicle," said Spencer Quong, senior vehicles engineer at the research group, in a telephone interview.

FEDERAL OVERSIGHT

The automakers' suit, filed in 2005 in US District Court in Burlington, Vermont, also challenges whether the state effort trespasses into an area overseen by the national government. Industry executives said the amount of CO2 emitted by a car is directly related to the amount of fuel it burns, and noted that fuel economy is covered by federal rules.

"This trial has been about whether or not states have the authority to set their own fuel economy standards," said Charles Territo, a spokesman for the Alliance of Automobile Manufacturers, a party to the suit, in an interview.

Vermont, backed by environmental groups, argued automakers have not explored all options in cutting CO2 emissions.

The automakers have said the new standards would require vehicles to be lighter, which could make them more dangerous for passengers in accidents.

But David Greene, an energy policy analyst at Oak Ridge National Laboratory, testifying as a private citizen, said his research had shown lighter autos are not necessarily less safe in traffic accidents, since over time all vehicles would become lighter to meet the new standards.

"There is, in fact, not a correlation between fuel economy and injuries, provided, of course, the fuel economy standard is within reason," Greene said. "That should be possible to achieve without a negative effect on highway safety."

District Judge William K. Sessions limited each side to 40 hours to make their case.

California was the first US state to act aggressively on auto emissions. US environmental laws have allowed the West Coast state to adopt stricter auto emissions controls than set by federal law. Other states can follow its lead.