Over the years, the U.S. Environmental Protection Agency has tested myriad gas-saving devices that burst onto the consumer scene: devices that bleed air into the carburetor or bubble air through a container of water and antifreeze mixture, fuel-line gadgets that heat the gas before it enters the carburetor, magnets that clamp to the inside or outside of the fuel line to change the gasoline's molecular structure and metallic fuel-line additives with dissimilar metals that claim to ionize the fuel.

Experts say they all have one thing in common. "They don't work," says John Millett, spokesman for the EPA. "Believe me, if it were that easy, cars would be built that way, especially the magnets and whirligig devices. It's smart to be skeptical about any claims like that."

The EPA to date has tested in the neighborhood of 100 gas-saving devices, the most recent at the request of the Federal Trade Commission, and only six "indicated a very small improvement in fuel economy without an increase in exhaust emissions."

Another four also made the itty-bitty improvement cut, but per federal regulations, the exhaust emission trade-offs mean consumers who slap these on their cars could face charges of illegal tampering.

Gopal Duleep, managing director of Energy and Environmental Analysis Inc., a technical consulting firm in Washington, D.C., estimates that 95% of the aftermarket products don't really change fuel economy.