FYI - You can control just about any PWM motor speed controller safely using Arduino with optoisolated safety - I have achieved this. The Arduino can read from an auxiliary 1 bar MAP sensor and respond accordingly with its own PWM signal to the mosfet on the motor speed controller using a pretty simple algorithm. This way it is easy to turn the system off at idle and the ramp up on acceleration to some pre-defined ceiling.
Doubtful you will ever be able to efficiently make enough HHO to keep up at full throttle even with batteries filling the trunk. At a certain point, weight will become a huge issue. There is a cutoff point in the load range of every automotive ICE where energy demand is far to high to be feasible which is why many people believe they have been cheated and the whole thing is a scam and everyone associated with this tech is a liar. You simply cannot floor it all day and expect a system like we build (which requires more energy in than you get out) to ever do more than waste gas.
Driving style affects the amount of air and fuel necessary to get from A to B. Drive conservatively and get good MPG. Drive conservatively with an efficient, properly conditioned and installed system that has been debugged for your specific vehicle and potentially get better MPG. Drive like racecar driver and get worse MPG. This isn't NO2!