The Obama administration says gas stations can start selling fuel with more ethanol _ a mixture of up to 15 percent _ but it's only recommended for cars and light trucks built since 2007.
Motorists with older vehicles will need to watch what they're pumping or risk damaging their engines with too much corn-based fuel, the Environmental Protection Agency says. The current blend is 10 percent ethanol.
The move means that gas stations, if they choose to sell the fuel, will have to use special pumps and signs to make sure consumers don't fill their vehicles with the wrong fuel. The EPA said it will propose new pump labeling requirements to help consumers figure out which gas to use.
The ethanol industry says the agency should have allowed the higher blend for more vehicles, arguing that testing shows that it is safe. The EPA was more cautious, saying Wednesday that it will wait until more tests are completed in November to approve vehicles manufactured between 2001 and 2006.
The agency said owners of cars and trucks made before that _ along with motorcycles, heavy-duty vehicles or non-road engines _ will have to wait even longer, if such vehicles are approved at all.
The move, which comes less than a month before November's midterm elections, is politically popular in rural farm areas. But ethanol faces strong opposition from the auto industry, environmentalists, cattle ranchers, food companies and a broad coalition of other groups.
Opponents argue that the increase in production of corn and its diversion into ethanol is making animal feed more expensive, raising prices at the grocery store and tearing up the land. Manufacturers of smaller engines _ used in everything from lawn mowers to boats _ also oppose increasing the use of the fuel, saying those engines are not designed for the higher concentrations.
The Obama administration has remained supportive of the renewable fuel. EPA Administrator Lisa Jackson said Wednesday that "wherever sound science and the law support steps to allow more homegrown fuels in America's vehicles, this administration takes those steps."
The EPA has said a congressional mandate for increased ethanol use can't be achieved without allowing higher blends. Congress has required refiners to blend 36 billion gallons of biofuels, mostly ethanol, into auto fuel by 2022.
The ethanol industry has maintained that there is sufficient evidence to show that a 15 percent ethanol blend in motor fuel will not harm engine performance. They say increased consumption of the renewable fuel creates new jobs and replaces imported oil.
The industry group Growth Energy petitioned the EPA to raise the blend in March 2009. The decision was initially expected last December but was delayed twice as the agency and the Energy Department completed additional testing.
Growth Energy President Tom Buis called the move a "good first step" and said the fuel could be available for sale as soon as early next year. He urged the agency to quickly approve the blend for older vehicles, saying there is no reason to limit the higher concentration to the newer models.
Critics said the decision could be a frustration to drivers and argued that many retailers will opt not to sell the higher blend because of the expense of adding new pumps and signs.
"We're really going to make the consumers a guinea pig here," said Craig Cox of the Environmental Working Group, an environmental advocacy group that opposes increases in the fuel. "Have we really thought through what it's going to take to distinguish E15 to E10?"
The EPA said the 2007 and newer models eligible for the fuel represent more than a third of current gas consumption and more than 65 million vehicles. The rules would affect only those cars and trucks because they have more durable emissions systems. Ethanol burns hotter than gasoline, causing catalytic converters, which treat engine emissions, to break down faster. The newer vehicles have components in the emissions systems that are better able to adjust to the higher ethanol blends.
Automakers said they were worried the EPA decision would eventually lead to motorists unknowingly filling up their older cars and trucks with E15 and hurting their engines.
The problem could be exacerbated if E15 fuels are cheaper than more conventional blends, prompting owners of older vehicles to use the fuel despite the potential engine problems.
"Anytime you're dividing up model years and having different fuels for different model years, then it risks a misfueling situation and that's a real concern for the longterm success of alternative fuels," said Gloria Bergquist, spokeswoman for Alliance of Automobile Manufacturers.
Since 2000, U.S. automakers have increased production of so-called flex-fuel vehicles, which can run on blends of up to 85 percent ethanol and 15 percent gasoline. There are currently about 8 million flex-fuel vehicles on the road. Detroit automakers have pledged to make about half of their vehicle production capable of running on E85 by 2012.
The Obama administration's decision to boost the ethanol concentration in gasoline is a victory for the industry as it struggles to hold onto other subsidies. An increased public skepticism of the renewable fuel has caused some lawmakers who have always championed ethanol to divert the money to other priorities. A key tax credit is scheduled to expire at the end of this year, and some in Congress are considering cutting it or doing away with it altogether.
Ethanol producers say expiration of the credits, which are paid to oil companies as an incentive to blend gasoline with ethanol, could mean the loss of almost 40 percent of the industry's plants and tougher times for a domestic fuel that is good for national security.
Associated Press writer Ken Thomas contributed to this report.