More Miles Per Gallon, Please

The scientific explanation

• Promoters claim fuel-line magnets will break up "clumped" fuel molecules so that gas burns more efficiently. "But gasoline molecules don't clump up," Allen says, "and … don't respond to magnetic force."
  
  

• Engine ionizers clip to spark plugs, supposedly to increase combustion efficiency. But in tests, Allen says, increased combustion served to decrease power — and triggered an engine fire.
  
  

• Vortex generators are said to mix fuel more efficiently with air. What they really do is reduce the air flowing into the manifold, reducing power.

• Vapor injectors are said to convert raw fuel to vapor outside the engine for better performance. In reality, engine computers prevent any such benefit.
  
  

• Water injectors use technology that provided emergency power in World War II planes. But tests show this technology doesn't work in automobiles.
  
  

• A device that plugs into a cigarette lighter is claimed to "smooth out noise" in electrical systems and increase mileage. No such gain was found in tests by Popular Mechanics and Consumer Reports.
  
  

So what does the device do? "It lights up when you plug it in," says CR's Sue LaColla.

Sid Kirchheimer is the author of Scam-Proof Your Life, published by AARP Books/Sterling.