The idea that we can fill up our tanks with tap water and drive around the same as we do today is pure urban legend. Or scam, depending on whether or not you've bought one of these "inventions."

It's a nice dream, and in desperate times desperate people will do desperate things. But it's just not going to happen.

Wikipedia's entry for water-fuelled car nicely debunks the types of "inventions" that have claimed to use water for fuel. Of the types of schemes come up with, the only one that's remotely plausible has water electrolyzed for its hydrogen and oxygen, but that would require a chemical at greater cost than gasoline or power greater than that which would simply run an electric car.

Also a buddy of mine posted something on a political board about a supposed super engine that people claim would get hundreds of miles to the gallon if only the world conspiracy holding it down could be broken. Just Google "hank benson" "super engine" and go to the Page 3 link, it's the fourth post down. It's not exactly about water fuel, but I think the ideas certainly apply.

Water for Fuel? The answer is no.