Armed with a new showing that as many as 15 percent of the post-1974 car owners on the road have used or are illegally using cheaper leaded gasoline in their cars, the Environmental Protection Agency is considering a fine against motorists who switch fuels.

According to Ben Jackson, an EPA Acting Deputy Administrator, the fines are one of four possible actions the agency is considering in an effort to curtail fuel switching.

Any EPA attempt to influence fuel pricing will be criticized by the Department of Energy, which has endorsed total deregulation of motor gasoline at the pump.

The new development as EPA are just the latest round of a disagreement between that agency and DOE over the cause and effects of fuel switching. When DOE first proposed gas deregulation, the EPA contended that the price differential between unleaded and leaded gasoline would grow larger than it has already become, thus causing more people to switch to the cheaper leaded gasoline. This is turn would pollute the air with lead.

EPA officials have contended that gas stations in some parts of the country are charging as much as 10 cents a gallon more for unleaded gasoline than leaded. Whenever a motorist puts leaded gasoline in a car that must, by federal law, take unleaded fuel, the gas station owner is subject to a fine of up to $10,000, but the driver is not subject to any fine.

According to the EPA some 40 gas station owners have been hit with fines averaging $1,000. But in many cases, the agency contends, the switch occurs at a self-service pump, which is virtually unwatchable by the station owner.

The four plans currently under consideration by the EPA include:

Imposing civil penalties on motorists who use leaded gasoline in cars meant for unleaded.

Require automakers to install and sell tamper-proof filler outlets. The present retainers on post-1974 cars are easily removed by motorists, thus allowing the leaded gasoline nozzles, which are too wide to fit through the retainers, into the tank.

Begin a nationwide inspection and maintenance program similar to some state systems used to guage emissions from autos.

Fix the differential price between leaded and unleaded gasoline - perhaps as low as zero, or one cent. While the actual price could vary, the difference between the two types of gasoline would vanish, thus removing the incentive to use the dirtier leaded gasoline.

"We now have unleaded gasoline subsidizing leaded gasoline," said Jackson. "If you were to control the difference between the two at zero, leaded would be subsidizing unleaded, which would make more sense in terms of the dirtier gasoline funding the cleaner alternative."

Jackson said a "significant number" of catalytic converters in autos that have experience switching are now burned out, or neutralized, by the use of leaded gasoline, and would have to be repaired before they could meet emission control standards should a national testing program be implemented.

The new EPA study points out that switching 'is found to be more prevalent in the Pacific region than in the East, in rural than in metropolitan areas, and among the younger, middle income, better educated segments of the population."

But DOE sources say that doubt EPA's logic in attempting to stop fuel switching. "People who are switching are the people who are price motivated," and often have lower income," said a DOE source.