March 27 (Bloomberg) -- General Motors Corp., Toyota Motor Corp. and other large carmakers may have to sell more plug-in electric autos in California as the state adjusts exhaust rules to account for slow development of newer types of clean engines.

California's Air Resources Board today proposed cutting the number of required battery-only and hydrogen fuel-cell autos sold in the state to 2,500, a tenth the initial requirement, in model years 2012 through 2014. The six biggest carmakers would instead have to sell a combined 75,000 plug-in hybrids and cars that burn hydrogen instead of gasoline.

Any changes to the emissions rules would influence the types of advanced cars GM, Toyota, Ford Motor Co., Chrysler LLC, Honda Motor Co. and Nissan Motor Co. sells in California, which accounts for 10 percent of all new autos sold in the U.S. A shift in California also would affect carmakers' sales in the 12 other states that have joined the Zero Emission Vehicle program.

``Fuel-cell vehicles are farther from commercial readiness than expected, due primarily to durability and cost hurdles,'' ARB researcher Analisa Bevin said at a hearing in Sacramento. ``Battery electric vehicles also continue to face cost concerns.''

California has forced carmakers since 1990 to offer vehicles powered by batteries, hydrogen or a blend of electricity and gasoline to curb exhaust.

More than 200 representatives of automakers, advocates for battery-powered cars and hydrogen fuel-cell companies gathered in Sacramento for the public hearing, the first review of the ZEV program in five years.

Driving Range

Hybrid gasoline-electric vehicles increase fuel efficiency by combining a battery-powered electric motor and a gasoline engine. Plug-in vehicles can be charged at an electrical outlet, and offer a greater electric driving range than conventional hybrids such as Toyota's Prius.

Plug-in hybrids, such as GM's proposed Volt model, can travel at least 40 miles solely on battery power before needing to be recharged at an electrical outlet. The model can also be recharged by an on-board motor powered by fuels such as gasoline. GM plans to sell the Chevrolet Volt as early as 2010.

Toyota announced plans in January to lease plug-in cars to U.S. fleets by 2010.

California, alone among U.S. states, has authority to set air-quality rules that exceed federal standards. The state has done so since the 1970s, when it initially required the use of catalytic converters to help reduce smog-forming auto emissions.

California regulators today also proposed changing requirements for the 2015 through 2017 model years, reducing the number of fuel-cell and battery-only electric cars to 25,000 from an initial target of 50,000. Additionally, carmakers would have to sell a combined 83,000 plug-in or hydrogen-burning vehicles. The plug-in cars would have to be able to travel at least 10 miles solely on battery power.

To contact the reporter on this story: Alan Ohnsman in Sacramento at aohnsman@bloomberg.net

Last Updated: March 27, 2008 18:44 EDT