There's more than fuel that you're pumping into your tank

WEST PALM BEACH, Fla. - WEB EXTRA: Click here for an interactive chart that tracks the cost of gasoline over the past 10 years.

When you go to fill up your car, what are you thinking?

"My concern is price, only price," said one motorist.

Other drivers do think about what they're pumping.

"Additives, detergents and reliability," said another motorist.

So what's in the gas that goes into your car?

Working with our partners at the Scripps Howard News Service, we collected one-gallon samples of regular, unleaded and premium fuel from 8 national brands across 3 states. We took our samples to Paragon Laboratories, an independent, certified testing facility near Detroit to find out if all brands are created equal when it comes to detergent.

"One of the worst things you can do is to go from brand to brand to brand, getting the cheapest stuff you can find," said Tony Molla of the National Institute for Automotive Service Excellence.

He says if you repeatedly fill up with gas that doesn't have enough detergent, deposits can build up causing your engine to burn more gas.

"As the deposits build up, they tend to attract more deposits, so it's very much like a process that the longer it goes untreated, the worse it becomes," he said.

Paragon performed unwashed gum tests. Those are the same tests automakers use when spot-checking to see if gas meets their standards. Scientists boiled each sample and measured the residue left behind; the more residue - the more detergent - which could equal better gas mileage.

In our test of regular unleaded gas, Exxon had the highest level of additives, with 20 milligrams of residue per 100 milliliters. BP and Shell followed close behind, while Marathon, Citgo, Pilot and Speedway samples each had less than half the amount of detergent found in the top three brands.

"Obviously the more detergent, the more efficient job it's going to do of cleaning the valves," said Molla.

You pay a premium for gas that's 92 or 93 octane, and in our test, Shell leads the pack with 31 milligrams. It's followed by BP and Exxon. We found the lowest levels of additives at Speedway, Mobil, Citgo, Marathon and Pilot. Pilot's 8.8 milligrams is three times less than Shell's.

In a statement, Pilot Vice President Alan Wright said, "Our gas blends meet EPA requirements. We don't put in extra."

"We always recommend that you try to find one brand of gasoline that you like and stick with it or at least one location," said Molla.

It's just one more thing to think about next time you're filling up at the pump.