It seems to me that the best way to make the computer deliver less fuel in every throttle position and throughout the entire RPM range is to change the air/fuel ration in the EPROM. Just a guess, but a change from 14.7:1 to 15.7:1 at sea level would facilitate a leaner mix.
What happens if the HHO delivery systems fails? The computer would have to sense this somehow and default back to the original ratio.
I've been calling companies that sell aftermarket PCMs and repair OEM PCMs and nobody wants to mess with the universal constant of 14.7:1.
Anybody know of anyone that can rewrite a few lines of code? It's just software.