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.'"