Using high-octane premium-grade gas in an engine that does not require it offers no benefit. But in engines designed to squeeze the fuel-air mixture to very high pressures before igniting it with the spark plug, high-octane fuel burns predictably and can produce more horsepower. (On the other hand, burning low-octane gas in an engine tuned for premium grade can cause erratic combustion, or knocking, and result in severe engine damage.)

Ethanol contains only about two-thirds as much energy as gasoline, gallon for gallon. But if it is burned in engines designed for high cylinder pressures, it will produce competitive horsepower.

In general, the oil companies have opposed using higher concentrations of ethanol. The oil industry is trying to get Congress to change federal rules so they can use less ethanol, not more.

But various engine and fuel experts like the idea, because the E.P.A. is inviting the auto companies to take advantage of the good characteristics of ethanol, including an octane rating that is well over 100.

“That’s getting smarter,” said Margaret Wooldridge, a professor of mechanical engineering at the University of Michigan. The way ethanol is used now, she said, “if anybody does notice there’s any ethanol in the fuel, it’s always in a way that is negative.”

The trouble with the flexible fuel vehicles on the market now, which can run at blends of up to 85 percent ethanol, is that they are still mostly optimized for gasoline, not ethanol, she said. While there are millions of such vehicles on the road, they run mostly on E10 because that is a better bargain for the driver.