The federal government deserves at least some praise for developing new gas mileage rules for light trucks and most SUVs. For too long under the federal government's Corporate Average Fuel Economy system, these vehicles were not adequately covered.

Under the new rules, automakers must meet an average 21.6 mpg for the 2006 model light trucks. That average will rise to 22.2 mpg for 2007 vehicles and up to 24.2 mpg by 2011. The new regulations will not affect passenger cars, which now must average 27.5 mpg.

These modest increases in fuel efficiency are steps in the right direction. Motor vehicles account for the lion's share of air pollution, and the most practical way to reduce emissions is to use less fuel.

Unfortunately, there is a major problem with the federal government's new fuel efficiency rules. They could supersede, and thus invalidate, even stricter standards in California and 10 other states that have adopted the California rules. They are: Maine, Vermont, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Washington and Oregon.

California air regulators approved rules in 2004 to cut exhaust from cars and light trucks by 25 percent, and from larger trucks and SUVs by 18 percent, starting in 2009. Those rules would result in a much greater reduction than under the new federal standards.

That is why 21 senators formed a bipartisan group to ask the federal Environmental Protection Agency to let California go ahead with its stricter regulations. Then other states could opt to follow the California or the federal rules.

EPA spokesman John Millett had no immediate comment, saying that the agency still was reviewing the waiver request. However, the situation may be out of his hands. The National Highway Traffic Safety Administration argued that Congress has "expressly pre-empted" states from issuing their own rules on mileage standards, and that any state law limiting carbon dioxide emissions would be disqualified.

It would be a mistake for a modest federal fuel-efficiency rule to negate more aggressive state regulations that should be allowed to go ahead in 11 states, four of which (California, New York, Pennsylvania and New Jersey) are among the 10 most populous in the nation.

By overruling the strict pollution regulations in these states, the net effect of the federal rule could be an increase in motor vehicle pollution, particularly if other states wanted to copy the California standard.

Let's hope the bipartisan effort by the 21 senators will bear fruit and the Bush administration will find a way to allow states to improve on the federal regulations.