In this age of $1-a-gallon gasoline, it's tough on motorists who have to shun regular gas and buy the more expensive unleaded fuel required in all recent American-made and many foreign cars.

Playing on consumers' inevitable desire to save money at the gas pump, several foreign auto manufacturers whose products still can use the cheaper regular gas have opened advertising campaigns to tout that fact and promote sales.

But every silver lining has its cloud: What you may save in gasoline costs you may lose in gasoline economy.

"Promoting the fact that a car can use regular gas is basically nothing more than a sales gimmick," says Charles Gray, Ann Arbor, Mich., director of emissions control technology for the Environmental Protection Agency.

"The only way a car can use regular gasoline is if it doesn't use a catalytic converter to control emissions. The foreign companies that don't use converters have to de-tune their engines to meet U.S. emissions standards, and that process costs them a substantial amount in fuel economy.

"The cars involved are still economical because they're small and light. But they could be substantially more economical if they used catalytic converters."

Exactly how much more economical Gray isn't sure, but he notes that when General Motors started using catalytic converters on its new-car fleet in the 1975 model year, the average fleet fuel economy increased by 25 percent.

Among the foreign cars that don't use catalytic converters are Subaru, Honda, Volkswagen and some models in the Datsun line. If one of those cars currently rated at 25 miles a gallon could get 25 percent better gas mileage with a catalytic converter, it would raise the MPG rating to 31, an increase that would more than offset the nickel- or dime-a-gallon difference between leaded and unleaded fuel.

"What American manufacturers do is tune all the variables -- carburetion, spark timing, everything -- to optimize engine economy," Gray says. "But the engines don't run very clean that way. So they shift the entire burden of cleaning up emissions to the catalyst. The converter lets them build an engine to optimize economy without worrying about pollution.

"The foreign manufacturers who make only small cars don't have to worry about meeting mileage standards, so they can de-tune their engines for optimum pollution control and sacrifice a few miles a gallon. It doesn't matter to us (EPA) which way the emissions are cleaned up, as long as the job is done."

However, the next two years will bring such stringent new levels of pollution control that Gray believes new cars without catalytic converters will be virtually non-existent by 1981.

Volkswagen, for example, is bowing to the inevitability of converters on all its 1980 model cars.

"We've had testimony from foreign manufacturers that they're paying a 20 to 30 percent fuel economy penalty by not using catalytic converters," Gray says. "Meanwhile, fuel economy on American cars has been improving steadily, with a lot more improvement yet to come.

"If the foreign cars continue to sacrifice economy for emissions control, they'll get to the point where they won't be able to get away without catalytic converters through the 1980 model year, but the carbon monoxide emission standard drops so much in 1981 that I expect that year will make converters a universal piece of equipment."