Additive marketers often state the fuel-saving effects will not become apparent until the product has been used for several tanks of fuel, and all of the companies require ongoing use of their product to maintain the benefits.

Manufacturers also often claim their product has been tested and registered with the Environmental Protection Agency.

"This is true," says Nielsen, "but the procedures they cite are mandated by the EPA before any fuel additive can legally be sold in the United States. The tests only prove the additive will not harm a vehicle's fuel system or increase the amount of pollution its engine emits; they do not address a product's effect on gas mileage."

The EPA does have a test that can measure the effectiveness of any fuel-saving additive - the same federal exhaust emission and fuel economy test used by vehicle manufacturers to certify new vehicles sold in the United States.

Any maker of a "fuel-saving" product can hire an approved independent testing laboratory to perform back-to-back EPA tests of the same vehicle, with and without the additive, to generate scientifically valid figures that will support their claims. To date, AAA has not found one manufacturer of a "fuel-saving" additive that has done so.

The most effective ways to save fuel are to have your vehicle serviced regularly at a AAA Approved Auto Repair facility and modify your driving behavior, Nielsen says. Other fuel-saving tips are: Keep tires properly inflated, remove unnecessary weight from the vehicle and carpool when possible.