It seems too good to be true, but rigorous tests under way in Nevada, California and Illinois show a breakthrough fuel that is more than half tap water could power the nation's vehicles, trains and gas-powered aircraft by century's end. The milky fuel was developed by Reno inventor Rudolf Gunnerman and is being pushed through the federal fuels-testing labyrinth by Gunnerman and diesel giant Caterpillar Inc. It has passed every test thrown at it. In virtually all categories, it tops conventional gasoline and diesel as a clean, cheap and safe fuel that can be used in almost any combustion engine. If it works -- and disinterested outsiders who have tested it say it may -- drivers could see the price of gasoline cut more than half. "Everybody said it cannot work, that I'm a fraud,'' the German-born inventor said. No one's laughing now: Nevada last November certified the water-based fuel as a "clean alternative fuel,'' meaning it can be used to meet federal mandates requiring clean fuels in fleets and other vehicles. The Energy Department is awaiting test data from trials run by Caterpillar before passing judgment. If DOE reaches the same conclusion as Nevada, Gunnerman's concoction could be used as a clean fuel in all states.