Do most gas-saving devices really work?

This category of devices includes vapor injectors, fuel and engine ionizers, fuel-line magnets and metallic catalysts inserted into the fuel tank. All of these devices are marketed on the claim that they cause the fuel to behave differently, essentially making it possible to burn fuel with more efficiency.

Some of these devices use magnets, electromagnetic fields or so-called "ionic corona generators" to allegedly change the molecular structure of the gasoline or diesel fuel. These products' manufacturers claim that this change makes fuel burn faster and more completely in the combustion chamber.

Other products claim to have a similar effect by injecting fuel vapor into the engine upstream of the fuel injectors. The claim behind these devices is that they introduce fuel into the engine in a more burnable state.

Beyond the question of whether these devices actually can change the composition of fuel, the idea of making fuel burn faster or better is a bit of a stretch. The fuel burn in most modern cars is optimized to the point where only a small percentage of the injected fuel leaves the engine without being ignited. Even if these devices did improve fuel burn, the improvement would not produce the mileage boosts of 9 to 13 percent, as some products' Web sites claim [source: Tony's Guide to Fuel Saving Gadgets].

Another factor to consider is that devices such as vapor injectors are essentially overridden by the engine's computer. Adding extra fuel through a vapor injector can cause the engine to reduce fuel flow from its injectors. The engine adapts to the new fuel flow, with little -- if any -- change in its performance [source: Allen].

The variety of additives, octane boosters and fuel system cleaners is staggering; it's easy to find the long racks of these products in any auto parts store. Some claim to clean carbon deposits out of your engine, while others claim to enhance fuel or oil performance through a variety of chemical reactions. Still others claim they coat engine parts, reducing friction and extending engine life.

It's hard to lump all of these additives into the hoax category, since some do indeed clean internal engine parts, and others may contain lubricants that don't harm the engine. But be wary of putting an additive into your car without first checking its contents: Some additives can damage modern cars' engine sensors, leading to expensive repairs. Additives are all different, so you're wise to research anything you plan to put in your gas tank or oil pan [source: Tony's Guide to Fuel Saving Gadgets].

In a world of mileage-hoax gadgets, are there tips, techniques or devices that will actually improve your mileage? There are, according to the U.S. Environmental Protection Agency and the U.S. Department of Energy. It may be surprising, too, that these mileage boosters often require little or no investment.

Proper car maintenance is a major key to getting the best mileage that your car is capable of achieving. A smooth-running engine and well-lubricated drivetrain not only keep your car running longer, but they also contribute to a more enjoyable driving experience. Consult your owner's manual to learn your cars recommended service intervals.

Driving techniques also play a big role in maximizing your mileage. Smooth stops and starts help you avoid wasted energy: A car burns more gas getting up to speed in a hurry than it does during gradual acceleration. Likewise, using the brakes in a sudden, jerky manner can waste energy, requiring more gas to get the car back up to cruising speed.

Finally, considering what's in your car can make a difference in how far your car goes on a gallon. A trunk or pickup bed full of heavy, unnecessary items hurts mileage. A simple thorough cleaning can do as much for your mileage in some cases as the latest fuel-saving gadget could ever hope to do [sources: U.S. Environmental Protection Agency, U.S. Department of Energy].

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