Thousands of motorists are using leaded gasoline in autos designed to use the environmentally cleaner unleaded gasoline because the low octane ratings of unleaded fuel causes major performance problems in cars, a gasoline marketing industry official says.

In a letter sent last week to Environmental Protection Agency head Douglas Costle, Independent Gasoline Marketers Council general counsel Jack Blum responded to an earlier question by Costle by saying, "We have no reason to doubt your numbers which suggest that somewhere which suggest that somewhere between 8 and 12 percent of all car owners are misfueling their vehicles."

But, Blum added, the fuel switching is more related to performance problems motorist are having with unleaded fuel in their vehicles than EPA's belief that the higher price of unleaded is driving people to use leaded gas instead.

"The key to fuel switching is performance," Blun wrote. "... the bulk of fuel switching is a direct result of the mismatch between the octane requirements of cars using unleaded fuel and the octane level of the generally available unleaded fuel."

Blum said that new cars coming off Detroit's assembly line "require 92-octane gasoline to operate at peak officiency."

But he added, "the commonly available unleaded regular gasoline is 87 octane. Fueling new cars with 87-octane unleaded gasoline creates unacceptable performance problems which are solved by fuel switching."

Blum said that for many major brands of gasoline, "the average octane of leaded regular is two points higher than unleaded and as a result some people switch from unleaded to leaded regular to improve automobile performance."

EPA had been at odds with the Department of Energy over DOE's plan to deregulate the price of gasoline at the pump, until the two reached agreement on Thursday to all limited price increases. Gas prices have been regulated by the federal government since the oil crisis of 1973.

But EPA's fears about deregulation have been based on its assumption that motorists are fuel switching solely because of the price difference between leaded and unleaded.

EPA fears, for example, that if prices are deregulated, the spread between the cheaper leaded fuel and unleaded fuel will increase to levels that will cause even further switching. Such switching to the dirtier leaded gasoline will increase pollution problems in concentrated areas, EPA contends.

But Blum's letter told Costle that although "your staff persists in playing down performance problems as an element in fuel switching, we believe that before you decide on adopting a price control remedy, you fully consider the performance problem and other appropriate remedies.