Over the years, the U.S. Environmental Protection Agency has tested a myriad of gas-saving devices that burst onto the consumer scene. These include 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, spokesperson 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 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 percent of the aftermarket products don't really change fuel economy.

Popular Mechanics magazine's experts tested seven fuel-saving products for its September 2005 issue and found no significant change in miles-per-gallon ratings. Two actually increased fuel consumption by 20 percent, according to the writer, and a third one melted before they could complete the test.

But none of this evidence stops an eager entrepreneur from pitching his product to people tired of watching the gas pump numbers spin.

"Over the last 15 years or so, I've seen the same products come and go under different names," says Roy Cox, manager of technical training and research for AAA Automotive and author of "Improving Fuel Economy: Money in Your Pocket."

Among the popular advertising claims the FTC is warning folks away from:

• "After installing your product on my car, I got an extra 4 miles per gallon." The trouble is, consumers aren't in a position to scientifically test their mileage for results. The testimonial may be heartfelt, but the driver didn't take into consideration traffic and road conditions, weather and his recent tuneup. • "This gas-saving device is approved by the federal government." According to the FTC, no government agency endorses gas-saving products for cars. The closest they can come to truth in advertising is to say that the EPA has reached certain conclusions about possible gas savings by testing the product or evaluating the manufacturer's own test data.