The Alliance of Automobile Manufacturers says it believes this could cause the converters, components that help control pollution, to fail at around 50,000 miles. They are supposed to last for 120,000 to 150,000 miles. “We are sensitive to the issues facing the ethanol industry, but the government must make decisions based on sound science,” said Dave McCurdy, president and chief executive of the alliance, in a letter to the E.P.A.

Another possibility is that the agency could waive the mandates requiring use of a large volume of biofuels. But that would anger farmers, who sell a great deal of corn to ethanol factories, and the members of Congress who represent them. It might also undermine the efforts of companies that are investing millions in factories to make ethanol from waste materials, like corncobs, straw and garbage.

“Ethanol is the only viable, competitive alternative to foreign oil,” said Tom Buis, chief executive of Growth Energy, the ethanol trade group that filed the petition with the E.P.A. to increase the blending percentage. “If we’re going to become less dependent on foreign oil, we’ve got to move forward.”

A third possibility is that the E.P.A. could announce that it is waiting for more data on how cars perform at higher blends, but that would merely put off the hard decision.

When Congress wrote the rules, in 2007, gasoline consumption had been growing for years, and it looked as if the nation would be able to use considerably more ethanol in the future. Gasoline consumption hit a peak of 3.4 billion barrels that year.

But gasoline demand fell in 2008, after soaring gas prices early in the year were followed by the economic crisis. Consumption was slightly less than 3.3 billion barrels last year, and it could end 2009 at about the same level.

With consumers buying more fuel-efficient cars these days, and carmakers rushing to bring even more of those to market, gasoline demand may not recover much in coming years, even as ethanol production soars.