Detergent in gas helps the engine run cleaner and smoother; so just how much is in there?

(WXYZ) - When you fill up your car with gas, do you always use the same brand of gasoline?

It does matter.

When we pump gas, we are putting more than just fuel into our vehicles. We are also putting in detergent that has been added to keep our engines cleaner.

Detergent in gas prevents carbon buildup which can impair engine efficiency, increase emissions and decrease gas mileage. 

Working with our partners at the Scripps Howard News Service, we collected one-gallon samples of regular unleaded and premium fuel from eight national brands across three states.

We took our samples to Paragon Laboratories, an independent, certified testing facility in Livonia to find out if all brands are created equal when it comes to detergent.

Paragon performed what is called unwashed gum tests, which are the same test 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 and that 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. 

You pay a premium for gas that is 92 or 93 octane, and in our test, Shell leads the pack with 31 milligrams, followed by BP and Exxon.

We found the lowest levels of additives at Speedway, Mobile, Citgo, Marathon and Pilot.

Pilot's 8.8 milligrams is three times less than Shell, but in a statement, company Vice President Alan Wright told us, "Our gas blends meet EPA requirements. We don't put in extra."

And a spokesperson for Citgo said the company ensures quality through random checks. 

Some experts say it is best to stick with the same brand of gasoline to be consistent with the amount of detergent going in your car.

Watch our story in the video player to learn more about gas detergent and to hear from the experts as they weigh in.

Some gasolines have earned distinction from TOP TIER Detergent Gasoline. Click here to learn more.

TEST RESULTS (Source: Paragon Laboratories)

Regular Unleaded 87 Octane

(per 100 mL.)

Exxon               20.0mg

BP                    17.2mg

Shell                16.2mg

Marathon          8.2mg

Citgo                 6.0mg

Pilot                   5.8mg

Speedway        5.2mg

Premium Unleaded 92 or 93 Octane

(per 100mL)

Shell           31.0mg

BP                26.4mg

Exxon          21.2mg

Speedway  10.6mg

Mobil            10.6mg

Citgo              9.4mg

Marathon       9.0mg

Pilot               8.8mg



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