Brand-name filling stations vs. convenience store outlets...

Dear Car Talk

Dear Car Talk | Aug 01, 1990

Dear Tom and Ray:

I own a two year old car. My friends have always told me that it's better to fill up new cars from brand-name filling stations. They say I should avoid convenience store outlets. Any truth to this?

RAY: Good question, David. The answer is yes and no. Most gas starts out the same. In fact, much of it comes from the same gasoline terminals. But by the time you put it in your car, it may be quite different. There are a number of things that can account for the differences.

TOM: The first is water. But not in the way you think. We've noticed there's a widespread fear of "watered down gas." Well, for the record, water and gas don't mix, so it's almost impossible for an unscrupulous gas station owner to stretch his profits by adding substantial amounts of water to the gas.

RAY: Water CAN get into the gas, however, in the form of condensation in the gas tanks. That tends to happen when the tanks are less-than-full for a long period of time. That's not an argument for or against convenience store gas, but it is an argument for buying gas where a lot of gas is sold. If a station's tanks are refilled frequently, condensation is less likely to build up.

TOM: The same argument applies because gasoline is also changed seasonally. It's blended differently in the summer than it is in the winter. In the summer, for example, gasoline suppliers decrease the volatility so that cars don't vapor lock. So if you're buying gas from Fred and Ethyl's Charm School, Deli, and Gas Station--a place that sells more gallons of milk than gasoline--you may be using gas that's out of season. Again, it's not that Fred and Ethyl started with inferior gas, it's just that it may be old gas by the time you get it.

RAY: Then there's the issue of octane. It's important to buy gasoline with the octane rating recommended for your car's engine. Lower octane can cause pinging, higher octane is a waste of money. Even though most states test for octane accuracy, it IS possible for an unscrupulous gas station owner to sell gas which actually has a lower octane than the rating on the pump.

TOM: Again, the fact that this practice has been known to happen doesn't necessarily favor brand name gas over convenience store gas. Since many filling stations are independently owned and operated, you could get tucked at a brand name station just as easily as at Fred and Ethyls.

RAY: The area in which brand name stations do appear to have a real advantage (leaving aside the issue of bathroom cleanliness) is in detergents and additives. While Fred and Ethyl may get their gas from the same terminal, most of the brand name gasolines blend in good quality detergents as they fill their trucks. Detergents help keep fuel injectors clean, and some detergents apparently are better than others.

TOM: The definitive study of detergents was conducted by BMW. Gasolines that got the highest ratings in their study were Amoco, Chevron, Exxon, Marathon, Mobil, Texaco, and Unocal. I guess Fred and Ethyl's must have just missed making the list.

RAY: So to answer your question, David, our advice would be three-fold. First, if you drive a fuel injected car, buy a high detergent gasoline. Second, make sure you buy a gasoline with the octane rating recommended in your owner's manual--no higher and no lower. And third, buy your gas where a lot of other get people theirs. Even it we're wrong about that, a least you'll have a chance to get some reading done while you're waiting on line.

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