I have learned that the computer measures the oxygen levels and adjusts the amount of gasoline to put in the engine with those sensors. Since I am pumping HHO, with is extra oxygen, the computer thinks they gasoline to air ratio is too rich in oxygen and dumps more gasoline in the system. So, that explains my results:
27 mpg with just baking soda
23 mpg with NaOH (1 tsp. drain cleaner), a better catalyst and did produce more HHO gas.

Normally I average 30 mpg.

Today I unplugged the O2 sensor on the front of the engine to see if there's a cheap way to trick the system. I suspect I will need to install the MAP sensor to trick the computer correctly.