jedi_sol -
If your 02 mod was something like an EFIE (
see here), then you may have been getting the MPG benefit from *only* the 02 mod because you are promoting a lean-burn condition.
I'm a believer that you can make HHO work, but this forum isn't the place for this discussion. You're better off going to a dedicated forum like
HHOINFO - The International OPEN SOURCE HHO/HOD Builders Network. .
CarloSW2