A friend of mine ran what he called a "hydrogen can" in his car for a while. Basically it was a jar of water with a long coil of stainless steel wire in it. He ran 12v through the wire straight off of the alternator and piped the gas that it made into the PCV valve. He claims that he gets about 10 mpg extra gas mileage.
...so with his old POS car that puts him at about 11 miles per gallon total, LOL.