AAA says some brands of gasoline may damage vehicle engines, decrease gas mileage

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With many people heading out for summer road trips, people are making their fair share of stops to fill up the tank.

But, not all gas is the same. Some brands may actually be decreasing your mileage.

So what is top-tier gasoline?

"Gasoline brands that meet top-tier requirements are those that have a higher detergent level than what EPA requires," says Mark Jenkins, AAA spokesman.

And those detergents clean off carbon deposits on your intake valves.

This is what an engine valve should look like, and this is one with carbon build up all over it. It needs a trip to the machine shop.

When these deposits get thick they restrict air to your engine, decreasing gas mileage and performance.

"Well eventually it will choke it down. Can you imagine a situation where you had to breathe through a straw and I asked you to run 100 yards? You're not going to be very competitive," says a mechanic.

AAA reports top-tier gas left 19 times fewer engine deposits after 4,000 miles.

So using the gas is kind of like taking most engines to the laundromat.

But if your vehicle has direct injection, like many newer cars, it doesn't matter.

"By having direct injection you don't have the benefit of the washing motion around the valve to keep it clean. It's kind of like washing your clothes with no soap. So it doesn't do anything at that point, it goes out and doesn't cycle and your clothes are still dirty," says Lee Williams, engine lab service manager.

AAA reports about one-third of our gas stations sell fuel with detergents, but Americans prefer location and rewards programs over quality.

Also gas stations using top-tier fuel cost about three cents more per gallon.

For clarification, top-tier gas has nothing to do with a gasoline's octane level. Retailers who sell top-tier gas must have the detergents in all grades of the fuel they sell.