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

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 none of these kits 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.

Most of 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 the engine runs on a slightly leaner mix of gasoline. But this small amount of gas savings is cancelled by the extra engine power needed to recharge the car battery as it produces the hydrogen, Ash said.