However inconceivable it may seem, ethanol has managed to earn itself another enemy.

To service-station owners, refiners, vintage-car collectors, economists and car manufacturers themselves, add motorcycle riders.

At issue is E15, the fuel blend consisting of 15 percent ethanol and 85 percent gasoline, rather than the typical 10-percent, 90-percent formulation, E10. The Environmental Protection Agency has approved E15 for all cars from the 2001 model year and newer, although most standard cars warn in the owner’s manual that using a blend above 10 percent can void the warranty. The E.P.A. has not approved its use for any motorcycles or all-terrain vehicles.

Last month a chain of service stations in eastern Kansas began offering E15 for passenger cars; higher-concentration ethanol formulations have long been available for so-called flex-fuel vehicles, which can run on any blend consisting of up to 85 percent ethanol. None of these, however, are intended for use in lawnmowers, leaf blowers or other gasoline-powered nonroad equipment, especially equipment that positions an engine close to the user, where a fire could be catastrophic.

To help keep E15 out of these and other engines, service stations offering E15 and E10 from the same pump station must agree to sell a minimum volume of fuel, four gallons, to the customer. The reason for the minimum purchase is to protect the engine of the next customer’s vehicle, as the fueling hose retains roughly a third of a gallon from the previous customer’s fill-up. Were the minimum-purchase rule not in place, a motorcyclist who followed an E15 customer at the pump, but who took only two gallons of E10, would receive a mixture that was nearly 11 percent ethanol. (The product of 1.67 gallons multiplied by 0.10 ethanol, which is 0.167, added to the product of .33 gallons multiplied by 0.15 ethanol, which is 0.05, would equal 0.217 gallons. That figure represents 10.85 percent of 2 gallons, rounded to 11 percent.)

But not all motorcycle fill-ups reach four gallons. Some motorcycles have tanks with that volume, but their owners may want to merely top them off.

Wayne Allard, vice president of the American Motorcyclist Association, noted the organization’s displeasure in a media statement. “The E.P.A. answer simply won’t work because of the sizes of many motorcycle and A.T.V. gas tanks and the fact that off-highway riders take containers of gas with them on their trips, and most times those containers are much smaller than four gallons.”

His group wrote to the E.P.A. on June 20, and Byron Bunker, acting director of the compliance division of the agency’s lab in Ann Arbor, Mich., responded in a letter (PDF) dated Aug. 1: “E.P.A. appreciates your concern about residual fuel remaining in a blender pump hose that dispenses E15 and E10 from the same hose.”

The agency acknowledged that owners of motorcycles and A.T.V.’s “should pay careful attention to the labeling of blender pumps to ensure that an appropriate fuel is chosen, in this case E10 or E15.” The agency added that it planned to conduct a “compliance survey” to ascertain whether the rule is enforced by service station owners.

“We will closely follow the results of the E15 Compliance Survey to determine whether additional misfueling mitigation measures are necessary,’’ it said.

Left unstated was how, exactly, a service station with pay-at-the-pump equipment would catch a customer who purchased less than four gallons and insist that the customer buy more.

However small the problem may seem, it has already generated ire on Capitol Hill. Representative Jim Sensenbrenner, a resident of Menomonee Falls, Wis., home of a major Harley-Davidson factory, said in a statement: “The E.P.A. has no business mandating how much gasoline Americans have to buy when filling up at the pump.”

“What if a rider doesn’t have a motorcycle with a four gallon tank,” he wrote. “Or if someone wants to fill a canister for their lawnmower or outboard boat engine, but it only holds two or three gallons? Or what if an American, struggling in this economy, just can’t afford four gallons of gas?”