One of the adds that popped up in gmail while sending out baby pics was to a web site that claimed you could run your car on water. I've always had a passing interest in perpetual motion machines and other claims that violate the laws of thermodynamics and everyone was napping, so I took the bait.
It is certainly oversold and borders on fraudulent, but it may very well improve the efficiency of your car. I'm still skeptical of those claims, but not on scientific ground, just on the level of hucksterism on the web page.
Lets start with the claim. Using just tap water, their system creates a special gas that can be burnt with gasoline in your engine. They have some fancy name for this gas. It is simply a mixture of Hydrogen gas and Oxygen gas in exactly the proportion needed to make water. This is done by pumping electric current through the water. It's a pretty well known process that may have been demonstrated in your high-school chemistry class, if your teacher, like our beloved Mr Merriweather, liked to blow things up.
The problem is that the energy needed for electrolysis is the same as the heat released when gas mixture is burned. Even worse, it is impossible to turn all the heat back into usable energy. So "Run your car on water!" is complete hogwash.
There are two points that might bail them out, however, since the fine print really only claims improved efficiency. First, the added hydrogen may improve the efficiency of the gasoline combustion by some process that I don't understand. The chemistry is beyond me, but I doubt its a big gain.
Second, and more significant, using the extra current from the alternator to build a reserve of hydrogen is analogous to the way earlier hybrids store energy in batteries to later boost the engine via an electric motor. This system is worse in that the combustion of hydrogen is less efficient than recovering energy from a battery and you probably don't have regenerative brakes on your car. On the other hand, the stored hydrogen won't leak energy and the system probably weighs less than the batteries and electric motor.