It's been a while since I've posted here, but here I go.
To get decent benefits from boosting a modern car, you have to trick the ECU to change ignition timing and cut back the fuel. You can mod your IAT sensor to retard the timing (make the ECU think the engine is running hotter than it actually is). Cutting back fuel is a bit tougher, however. To do this, you have to simultaneously trick the O2 sensor, MAF and MAP sensors. MPGResearch has details on how to do each of these mods. Note that if you only trick one of the sensors, the ECU will learn around your mods. All of them have to be done at the same time.
An electronic kit has been developed called the O2 Optimizer, which does all of this at once. It's being sold with the "."
MPG is guaranteed to go up by 50%. Many vehicles see a 100% gain.
No, I'm not advertising. I'm just pointing out the importance of tricking the ECU to take advantage of the hydrogen.
With the much more efficient electrolyzers that are made by people on this board, HUGE gains should be pretty easily attainable.