Keep in mind that hydrogen vehicles do not burn hydrogen in a combustion engine. Rather the hydrogen is in a fuel cell and when combined with oxygen it creates electricity - which powers the vehicle.
https://www.wisegeek.com/how-does-a-fuel-cell-hydrogen-car-work.htmThere are "hydrogen generator" kits which can be built and installed in vehicles. Using the charging system to extract the hydrogen from H20, the hydrogen is then fed into the combustion stream. However, there are some laws of physics out there that say you can't get more energy out of a process than you put in so its questionable as to whether there are any gains from this process. Like anything, there are people out there who've built and use these things and swear by them, I know a few. None of them are getting a hundred miles to a gallon in a full size pickup !!!
