Many people can already use hydrogen to run their car, even if as a hybrid. However electrolysis uses more energy than you would get out of the hydrogen it produces and under normal circumstances your average car battery wont allow you to produce as much hydrogen as you would like.

Toyota prius can run a car at 42mph max speed on electricity. Take all the features of that car that allow the battery to recharge & use it to create hydrogen via electrolysis & you would get much more power and use no fossil fuel.

This may be complicated to set up & would probably require reprogramming of the computers esp. for fuel injection so that you burn the hydrogen without any waste.

I've never liked to do math so if some one would like to take a guess at this

Energy loss creating hydrogen 20-40% and probably varies by device.

How efficient is running a car off pure electricity especially at highway speeds, my guess is not anywhere near as good as burning hydrogen. You would probably be better off using this technology to do a Hydrogen hybrid, if you cant find a way to run it completely off hydrogen using electrolysis. (such as better reuse of energy; Heat, exhaust, wind, etc.)

anyone here with more knowledge of this kind of stuff want to go over this thought & see if it's as plausible as it sounds? I'm not a scientist just a guy with an idea >.>