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.

The real scoop

Still, innovative breakthroughs happen every day in every field. Who says the gadget on the Internet isn’t that phenomenon?

“These vendors will probably tell you there is some sort of conspiracy among the automotive manufacturers, but nothing could be further from the truth,” Cox says. “They (manufacturers) do huge amounts of testing and put a lot of resources into research. It’s not something they take lightly.”

Consider the 1981 air-bleed device that the EPA discovered really did affect fuel economy positively. It shut the air-conditioner off during short periods of acceleration because the engine was already working hard. Today, new cars don’t need the product, because Ford, General Motors and the gang incorporated that winning technology into their designs.

But the chances of lightning striking twice like that, particularly with piston engines, don’t impress Duleep.

“It’s a very small chance, largely because the piston engine has been around for more than 100 years. During this time, every particular angle has been explored, and the chance of somebody overlooking something fairly obvious is pretty small,” he says.

For the auto-mechanically minded, it’s common knowledge that inefficiencies in the engine and transmission account for much of a car’s energy loss, and those are the areas where Duleep searches for improved fuel economy.

He likes GM’s Active Fuel Management engine that uses eight cylinders when you need peak power on an interstate entrance ramp, for example, but cuts down to four cylinders during normal driving conditions. (DaimlerChrysler calls its version the Multiple Displacement System.)

“Those are the kinds of things that help you reduce the 20 percent to 37 percent gap between maximum efficiency and the typical efficiency in a car’s energy use,” Duleep says.

Cox, too, points to gas-guzzling problems in his booklet: If carbon buildup or running a few degrees too warm causes the engine to knock, the powertrain control module is programmed to retard the ignition timing to correct the problem. However, this retarded timing reduces the engine’s power and, thus, burns more fuel. Conditions like combustion chambers that are too hot, worn piston rings, valves and gaskets or inadequate electrical power from the battery can also suck up to 8 miles per gallon off your bottom line.

Sorry, Charlie

But notice none of these areas experts cited has anything to do with fuel-tank additives. Most of these magic pills or sticks you drop into the gas tank claim to enhance performance by removing deposits.

“Only in very limited circumstances would that really help, because gasoline has detergents in it that take care of deposit buildup anyway,” says Millett.

Those limited circumstances involve an older vehicle — one with a carburetor rather than a fuel-injected system — that is driven infrequently or in an atypical way.

“But for the vast majority of drivers, they don’t offer a benefit that EPA has been able to say,” he says.

Gizmos that fall into the “vortex generators” category in essence create mini tornadoes out of the inlet air between the air cleaner and intake manifold.

According to Popular Science, the idea is to mix fuel with air more thoroughly so that it burns more completely in the combustion chamber. Yet the turbulence reduces the amount of air sucked into the manifold, putting less power at your fingertips. That’s how one of the brands Popular Science tested wound up costing a driver 20 percent more of his precious gasoline stash.

Cox has found another flaw with the bulk of air-injection products: They fail to combine air and fuel in the first place. “We don’t inject the fuel until it gets right down next to the intake of the actual cylinders in the engine. So there are usually at least a couple of inches between the air intake and the fuel,” he says.

Some car enthusiasts do purchase computer chips to recalibrate the engine and transmission, Duleep says. Such tinkering could cost up to $10,000, although you can find plenty of chips that supposedly upgrade your performance and mileage in the $200 to $300 range.

“You can see some modest fuel-economy improvements from those interventions, but it’s usually done for performance or to add extra cachet to your car,” he says.

Gas prices alone wouldn’t justify that mechanic’s bill.

Finally, the idea of applying magnets to the fuel line has captured many a car owner’s imagination. The EPA’s October 2005 reports, however, throw a wrench into that pipe dream. The product’s advertised 27 percent fuel-economy improvement didn’t materialize in the lab.

“The oil companies and automobile manufacturers are all saying the same thing, ‘We wish it was that easy — a pill in the tank or a magnet to paste into the fuel line — because we would really like to get 20 percent better fuel economy ratings,'” says Cox. “We’d be all over it.'”