DAYTON, OHIO - With gasoline prices at record highs, dozens of Web sites have appeared selling conversion kits that claim to improve gas mileage by as much as 100 percent.

With come-ons like "Run Any Car on Water, and ''Use Water for Fuel," the sites offer methods for injecting water gases into your engine, with different explanations of how this will improve your mileage. Prices for the kits range from $49 to several hundred dollars.

But automotive experts say the kits all have one thing in common -- none has been proven to work.

"These devices are a good way to make money, and that's why they're all over the Internet," said Steve Ash, head of the automotive technology department at Sinclair Community College in Dayton, Ohio. "If indeed they did work, they should have EPA-certified mileage tests to prove it."

The kits run a current from the car battery through a container of water to break it down into hydrogen and oxygen gases, Ash said. The hydrogen is inserted by tube into the car's air intake system so that the engine runs on a slightly leaner mix of gasoline. But Ash said this small amount of gas savings is cancelled by the extra engine power needed to recharge the car battery as it produces the hydrogen.

There is one way they can help, says automotive writer James Halderman.

"If you just bought something to improve your car's mileage for $100 or $200, you're going to drive differently" by easing off the accelerator on take-offs and lowering your overall speed.