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!
Stevo, you are right about full throttle. Fortunately most of us do not drive at full throttle for very long. Most people have been cheated because of a number of reasons. This is much more complicated than most think. The fact is still you need to keep the ratio of HHO to fuel/air at the optimum through all demand for your driving. If you are a racier you are not looking to get good mileage. LOL With the right quality of HHO the volume is lower than one thinks. I have found for the heavy footed drivers water injection is mandatory and advancing the timing instead of retarding it is possible with the water injection. Still the right amount of HHO at the right time is necessary. There are many methods to achieve this balance. I have found using the MAP of MAF signal can be a problem on some vehicles because either the driver or the signal is bouncing all over the place. This is only in some cases. This upsets the ratio/volume of HHO, resulting in poor results.