Do you think we could use water in our car changed to hydrogen to power our car?

Do you think we could use water in our car changed to hydrogen to power our car?
or would we have to have home hydrogen machines using electrolysis.i know the oil companies would complain

You can't start with water, make hydrogen and oxygen, to combine back into water without adding energy to the entire system. Especially not if you intend to use the energy of combining hydrogen and oxygen to power your vehicle. This is simple thermodynamics.

If we are to use hydrogen to power our car, that supply will have to come from outside of our cars. You suggest a home electrolysis machine.

While such a machine is feasible, there are some design considerations:
- The resulting hydrogen will need to be dried, cooled and liquefied for easy storage. This will take even more energy, which will be lost, making hydrogen even more costly.
- Hydrogen atoms are small, and so readily leak through many rubber gaskets, just as helium (which is bigger) escapes from a balloon without a hole in it. This makes storage challenging
- Your garage would need to be equipped with hydrogen detectors, lest you turn it into another Hindenburg. Homeowner insurance rates will rise.

The oil companies wouldn't complain. They'd just get into the hydrogen manufacturing business, to save you the hassle / danger. Most already have some kind of chemical manufacturing division already.


Oil refineries already make the equivalent of millions of gallons per day of hydrogen and they hold a lot of the patents on plants

Yes you can. Can you do it in a feasible energy efficient fashion? No!(unless you have your own private fission reactor)