Won't work - can't work - impossible to work.
It's called "the law of conservation of energy".
Energy in can not equal more than
energy out. Think of it this way:
1) You break water down into hydrogen and oxygen using power from your car's electrical system. This is the
Energy In stage.
2) You recombine the Hydrogen and Oxygen in the engine compartment. This is the Energy Out stage.
Whatever you put in, that's the most you can get out.
There is a
slight cooling benefit from pumping hydrogen into the cylinder, but it's pretty small.
If you don't like my physics explanation (which is 100% correct btw), think of it this way. If it really was as simple as adding a few hundred dollars worth of crap to your truck and gaining 2mpg, why wouldn't Toyota being doing it at the factory? BECAUSE IT DOESN'T WORK.
Read this:
SCAM ALERT - Adding Hydrogen To Your Gas WON’T Save You Money | Accurate Auto Advice
That's an even better explanation.