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Gasoline is expensive and you're looking for every way possible to save money at the pump. You already shy away from premium fuel, knowing that your car doesn't require it. You'd like to save a few pennies per gallon more by going to an off-brand gas station. But you can't get rid of the nagging fear: Is the cheap gas going to damage your car's engine?

Edmunds.com put this question to experts in several fields, including an automotive engineer at a major carmaker, gasoline manufacturers and two engineers with the American Automobile Association (AAA). It boils down to this: You can stop worrying about cheap gas. You're unlikely to hurt your car by using it.

Because of the advances in engine technology, a car's onboard computer is able to adjust for the inevitable variations in fuel, so most drivers won't notice a drop off in performance between different brands of fuel, from the most additive-rich gas sold by the major brands to the bare-bones stuff at your corner quickie mart.

Still, spending a few extra pennies per gallon might provide peace of mind to someone who just purchased a new car and wants to keep it as long as possible. People with older cars might not be as concerned about their engine's longevity. They can buy the less expensive gas and still be OK.

Steve Mazor, chief automotive engineer with the Automobile Club of Southern California, summed it up this way: "Buy the cheapest gas that is closest to you."

Recipes for Performance — at a Price
But this doesn't mean that all gas is the same, even though it starts out that way. The fuel from different filling stations comes from a common source: the "base gas" from a refinery. Workers there mix additives mandated by the Environmental Protection Agency into the base gas in order to clean a car's engine and reduce emissions. Then, the different gas companies — both off-brand and major brands — put their own additive packages in the gas to further boost both cleaning and performance.

A key difference is that the major brands put more additives in their gas and claim to have some secret ingredients. This extra shot of additives provides an additional level of cleaning and protection for your engine.

But is this extra helping of additives, which jacks up the price, really necessary? And, if you don't use more expensive, extra-additive gas, how soon will your engine's performance suffer?

"It's not like any of the fuels are totally junk," says John Nielsen, director of engineering and repair for the AAA. "If you buy gas from Bob's Bargain Basement gas station because that's all that's available, it won't hurt your car," he says.

The real difference is the amount of additives that are in the gas, Nielsen says. More additives essentially afford more protection — but they also cost more.

Some automakers and oil companies believe that the amount of government-required additives isn't enough to protect engines. They have created a Top Tier gasoline designation. It means that those gasoline brands sell fuels that provide more and better additives.

Nielsen recommends that drivers look in their car's owner's manual to see what the carmaker recommends and, when possible, follow that guideline. People who are still concerned about gasoline quality can ask a specific oil company if it has performed independent testing to substantiate its claims.

Selling the Secret Sauce in Gasoline
The major oil companies spend millions of dollars convincing buyers that their gas is superior by creating ads that feature smiling cartoon cars, lab-coated nerds and sooty engine valves. Buy Shell's nitrogen-enriched gas, for instance, and you won't get a buildup of "gunk" in your engine, company advertising promises.

Is all this just a marketing gimmick?

"I am a Ph.D. chemist, a nerdy guy who wears a white coat," says Jim Macias, Shell Oil Company's fuels marketing manager. "We really believe there are differences in fuels. We can see it, feel it and measure it."

Macias says the gunk caused by fuels with insufficient additives can foul fuel injectors and even trigger "Check Engine" lights in as few as 10,000 miles.

But not everyone is keen to talk about gasoline quality and whether additives really make the difference.

Edmunds sought comment from one well-known seller of low-price gas: Arco. Arco also often finds itself targeted as being a lower-quality product. BP, Arco's parent company, did not respond to Edmunds' interview request.

The American Petroleum Institute provided background comments about fuel additives and promised to provide an expert for an interview. The API spokesman never called back.

Finally, Southwest Research Institute in San Antonio, an independent, nonprofit testing facility, also declined to comment on the question of gasoline quality.

The Skeptics and Their Tests
The Auto Club's Mazor was more forthcoming, and has some interesting results from a blind test he did on three samples of gasoline from both major and independent gas stations.

"We tested emissions, fuel economy and performance and we could not tell the difference," he says.

Mazor believes that the driving public has outdated notions about gas. Twenty years ago, only premium fuel had detergents in it. Back then, it was beneficial to occasionally buy a tank of high-test gas to clean the engine. Then, he says, "regulations were very lax and there was little enforcement. But all that has changed."

Likewise, Randy Stephens, chief engineer for Toyota's Avalon, isn't wholly convinced by the claims of engine protection afforded by higher-priced gas. He says fuel experts at his company study the effects of different brands of gas on the Toyota engines. Automotive engineers disassemble engines after 10,000 miles of running them on different brands of gas to see if there is a difference.

"Honestly, in the 10 years I've been in charge of Avalon, I've never seen one come back with any sort of deposit issue," Stephens says.

Nevertheless, Stephens admits to being "swayed" by ads that tout cleaning agents. Twice a year he adds a bottle of Chevron U.S.A. Inc.'s Techron — the same additive that's in Chevron gasoline — to the fuel tank of his personal car.