Auto makers, environmentalists and the Obama White House reached an historic agreement two months ago, raising U.S. fuel economy rules and putting to rest years of bickering over California's right to set its own emissions standards.

But the state regulator in the nation's largest new-vehicle market continues to flex its administrative muscle.

The California Air Resources Board recently adopted a law requiring new cars sold in the state to use windows that reflect heat-producing rays from the sun. The rule takes effect in 2012, the first model year auto makers begin driving towards a 2016 fleet fuel-economy bogey of 35.5 mpg (6.6 L/100 km).

CARB says the measure will increase fuel efficiency and prevent the release of some 772 million tons (700 million t) of carbon dioxide into the atmosphere by reducing the need for vehicle air conditioning.

The technology exists and is in use in Europe. It will cost about $70 per vehicle to meet a 2012 standard in the U.S. and $250 to reach a stricter 2016 rule, CARB estimates, noting the extra expense would be recouped over five to 12 years.

Separate from the new corporate average fuel economy numbers California agreed to on May 19, the latest glass rule is another step by the state to independently regulate emissions. It comes on the heels of a new law calling for low-carbon vehicle fuels and a rule ensuring drivers measure their tire pressure during smog certification checks and oil changes.

On May 1, CARB shelved a controversial plan to mandate the phase-in of heat-reflecting paints beginning with the '12 model year. Paint manufacturers told CARB the concept was not technically feasible. But the regulator says it intends to move forward additional rules calling for low-rolling resistance tires and a standard for lower-viscosity motor oil.

CARB spokesman Stanley Young says rules such as the window glass regulation, as well as the new tire standard, could affect auto makers.

“Some will have to adjust their shipping and manufacturing processes,” he tells Ward's.

Auto makers vehemently opposed allowing states to set their own tailpipe standards, claiming it would cause a patchwork of costly regulations that could lead to poorer emissions standards overall.

General Motors Corp., a leading opponent of patchwork emissions rules, declined comment on the widow-glass regulation, referring Ward's to the Alliance of Automobile Manufacturers.

“Our hope is nothing distracts us from one national emissions standard,” says Wade Newton, a spokesman for the Washington lobbyist group. “In short, we are looking for advancements based on sound science — performance-based solutions without technology being dictated to us.”

Newton says the window rule, as it stands, represents the latter because it mandates that windshields must contain reflective glass. But the stricter 2016 measure calls for solar-absorbing glass and would reduce interior heat, taking into account all windows on the vehicle.

“We'd like it performance based from the beginning,” he says, noting auto maker resources are stretched thin given the nation's current record-low new-vehicle sales downturn.

But unlike the Clean Air Act waiver, which California sought to regulate tailpipe emissions and was denied by President Bush — later to be put under review by the Obama Admin., leading to the historic CAFE hike — the state does not need a federal exemption for its rules..