ROCK HILL, S.C. -- Willis Tarbox, 77, claims his old Ford pickup can go farther on a tank of gas than it used to. He once got 13 miles per gallon, but now he gets around 20.

Steve Anderson said his 2000 Ford F-250 diesel was also a nightmare to fill up twice a month.

“It costs about $160 every time I fill it up,” he said.

Last month, both men planted a small device under their hoods that they claim increased their gas mileage 30 percent. It's basically a jar filled mostly with water. There's a coil connected to wires, and baking soda, or something similar, which creates a charge and produces a small amount of hydrogen gas.

The hydrogen follows a path through a tube and into the air intake of the engine. Anderson said the added hydrogen causes the engine to burn less gas.

He said he was frightened by the idea at first.

"The first thing is, ‘Gosh, what happens if it blows up?’” he said.

But after six weeks, he's seen no problems and he said he averages around five miles per gallon more.

But can a kit you can buy online for $80 really do that?

Workers at the Rock Hill mechanic shop Premier Automotive wanted to find out.

"We've been testing about six to eight different types that you get off the Internet,” said mechanic Richard Garnett.

It's not a scientific test, but Garnett spent $2,000 of his own money ordering the kits from several Web sites. They were priced from $80 to $500.

For five weeks, he put them in cars and trucks, checked them daily, and logged the mileage He determined most of them are a waste of money.

Garnett said his tests showed some devices worked, but not for long.

"You get eight to 12 hours of run time, and then they burn up and go away,” he said.

But Garnett saw some potential, and he has been working for months to perfect a better design. In fact, he has a growing waiting list of customers wanting one installed.

However, the device may be no good on newer cars, which are loaded with computer-controlled sensors. Garnett said the sensors could react by sending more gas into the engine, giving you even worse mileage.

Another drawback is maintenance. You have to keep water in the jar and check the connections -- enough hassle to turn off most people.

Tarbox, who cruises around with a sales pitch on his car, had this to say to doubters who claim it's all a hoax, "When they make a blanket statement saying that none of them will work, they're lying."

Eyewitness News couldn't find a mechanic willing to install one right now, much less stand behind it. Also, mechanics warn if you're not comfortable working on cars, don't install one of these kits yourself. Garnett said the tests he did did not cause any engine damage, but drivers should talk to a mechanic they trust before looking into it.