Are you among the millions of Americans who like to “treat” their cars to an occasional tankful of premium gasoline? If so, you’re not only not doing anything good for your car -- you’re also contributing to a waste of some $2.1 billion every year.

That’s according to a new study from AAA, where researchers extrapolated from a consumer survey to determine that 16.5 million motorists filled up unnecessarily with premium gas an estimated 270 million times in the last year.

It seems an urban myth has become entrenched saying that fueling up now and then with gas having the highest-grade octane is a good idea, even if your car is designed to run on the regular stuff. This notion may have started in the 1950s, when premium contained a tetraethyl lead additive that prevented damaging engine knock (caused when fuel ignites prematurely in a cylinder’s combustion chamber) and helped engines run cleaner.

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It was promoted with slogans like: “Speedway is going steady with Ethyl.” Now, though, all grades of gas are required to have engine-cleaning additives -- excluding the toxic lead compound.

Having determined how many motorists were buying premium gas in place of regular, AAA continued with laboratory research to see if that premium gas conferred any benefits. Using a dynamometer -- essentially a treadmill for cars -- researchers determined that using premium in cars that require only regular did not increase power or gas mileage, nor did it cut down on polluting emissions.

“Drivers see the ‘premium’ name at the pump and may assume that the fuel is better for their vehicle,” said John Nielsen, AAA managing director of automotive engineering and repair. “Premium gasoline is higher octane, not higher quality.”

The AAA research came up with some additional findings.

70 percent of Americans drive cars that need only regular gas, while 16 percent require premium and the remaining 14 percent need mid-grade gas or have an alternative power source like electric batteries.

Modern engine-control systems can adjust automatically to deal with any problems from lower-octane gas, a capability not available in earlier decades.



Getting scheduled maintenance done in a timely manner will help keep gas mileage higher and help the engine run better.



AAA didn’t address the opposite premise: Will it hurt my car to save money by using regular when the manufacturer calls for premium gas?

But automotive website Edmunds.com has done research on the topic. Its take: If your owner’s manual says premium gas is “recommended,” using regular should be fine. However, if it says premium fuel is “required,” stick with that guidance. Steady use of regular gas could eventually damage such engines.

Edmunds has a list of cars for which premium is recommended and another for which it’s required. The latter runs heavily to German luxury brands like Audi, Mercedes-Benz, BMW and Porsche.