In the first place the car/truck is not running on water. The setup uses electrolysis to separate the hydrogen and oxygen in the water into a gas, which is fed into the engine. Yes your mileage will go up a bit because of the extra-oxidized fuel you are adding but not by much. And it's very tricky to get running consistent. The computer in cars now days tends to adjust all of the benefit out of the hydrogen/oxygen gas you add.
A normal gas engine will actually run worse when powered by hydrogen because it burns so much faster then gasoline. If you want the engine to run on a hydrogen oxygen mixture you will need to change the timing of your engine to 0 not 5 to 9 degrees BTDC. The timing on a gas motor is set BTDC because gasoline burns much slower than hydrogen/oxygen. You also have that big assed chunk of iron they call a flywheel because of the BTDC setting of a gas motor. If you readjust the timing for hydrogen then it won't run good on gasline and vise versa.
Don't get me wrong... it does work but the other problem with using the "fuel cell" as they call um is the rate at which the water is separated into the two gasses. The engine will suck up more than the cell can make. And a cell big enough to do the job of running the whole car/truck would be larger than would fit under the hood not to mention the amount of electricity it takes to run an electrolysis setup that big. Supposedly you can run for hundreds of miles on the power in one gallon of water but I never seen anything to confirm that. And trying to convert water fast enough to run down the hiway makes so much heat in the cell you end up boiling off most of the water.
The other consideration in having that much already mixed hydrogen/oxygen under a hood next to a hot engine is, it's very dangerous.
I tried a setup I built myself a year back and at best I could get a 4 to 6 mile increase in mileage but the damn thing was a royal headache to keep working properly.
I decided in the long run just keeping my foot off the gas pedal and driving a little more wisely saved me more and was far less hassle.
If you do decide to try it you may as well go buy you a 100 amp alternator in the first place cause the extra power draw will kill the stock one pretty quick. Also you will need a way to bypass any computer the car/truck has and set the timing on the fly. The parts are very easy to find and or build so if you wanna try it don't by one online. The markup in parts is like 600%
My Grandpa told me once that you have to learn by the mistakes of others because you will Not live long enough to make the all yourself.