They can take variety of shapes and forms:
- gas pills that you add to your gas tank
- alleged "power conditioners" you plug into your electrical circuit (usually cigarette lighter)
- fuel conditioner you clamp onto a fuel hose, usually a magnet of some sort
- air improver that somehow makes your air intake more efficient
They are ALL bull****. Why? Simple logic: if ANY of these devices actually save any fuel, don't you think places like taxi companies or shuttle van companies would be buying as much of it as they can? They spend far more time on the road consuming far more fuel than you ever will, and thus they would benefit far more from these devices than you can. Yet all of the alleged testimonials are from individuals.
In fact, everybody with ANY sort of scientific test have REPEATED busted ANY SORT of these fuel saver device claims. In 2005, Popular Mechanics tested a series of these products and found most of them REDUCED engine power and WORSENED fuel economy. Mythbusters also did a test and came up with similar results.
Gas Pills / Treatments / Etc. -- FAKE
Any sort of pill, liquid, or whatever you add to the gas tank will simply be burned, but fuel efficiency will not be affected by a TINY bit of chemical (remember, you have a 10-25 gallon tank, and how many pills or ounces did you put in?)
There are gas treatments, such as STABIL gas stabilizers that allows prevents gasoline from 'going bad' in vehicles that will be left alone for a long time. There are also "injector cleaners" that you can add to gas tank to clean your fuel injectors. However, they don't actually save any fuel.
Power Conditioner -- VERY FAKE
At least one power conditioner was revealed to be a total fake: its internal circuits are just stuff you can put together from parts at Radio Shack for $5 and have NO effect whatsoever on your car's electrical system.
Furthermore, the ignition circuit is usually isolated from the accessories circuit in the car, which would include the 12V aux power plug (i.e. cigarette lighter plug). Ignition is controlled by engine computer, plug wires, and the spark plugs themselves. Thus, anything plugged into the socket inside the cabin is unlikely to affect ANY of the engine electronics.
Even if somehow such a device can influence the spark plugs, a brighter spark does NOT automatically mean better power, as the ignition timing must be changed as well, and the amount of fuel injected, and so on. The computer must be re-calibrated as well.
You'd be better off replacing the spark plugs at your next tune-up. THAT actually can give you SLIGHTLY more horsepower... maybe 1-3 HP.
Magnetic Fuel Conditioner -- FAKE
Any sort of magnetic "fuel conditioner" is even more of a snake oil, since oil is NOT magnetic in any way, not is combustion. This supposedly "alignment" of particles is pure bull****. In fact, there is a full section on "magnetic scams" here that supposedly works on water, gas, fuel, and more!
If you believe in this sort of bull****, you can buy a more expensive "heavy-duty" version too. And I have a bridge to sell you.
Air Enhancer -- FAKE
Any sort of "air enhancer" is also bull****. Simple logic would tell you that the LESS obstructions you have in the air passage, the better the air will flow. Instead, these bull**** devices claim somehow that their devices, when added in the air intake as additional obstruction, will somehow make the air flow BETTER instead of worse, by generating vortices. WHY? You want a SMOOTH air flow into the engine!
There are real "air enhancers"... they are called turbochargers and superchargers and costs over one thousand dollars (more if you need installation). There is also K&N Filter and cone-style air intakes (sometimes called cold air intakes even though not all are). They work by providing a LESS restricted air intake path than the stock air filter. But adding something into the intake and expect that to improve air? You gotta be kidding me.
Don't get taken by these bull**** devices, even if they spent big bucks on sponsorships and are sold at major auto parts store. They don't work. End of story.