I run the AC 100% of the time (have a newborn in the car, so its not a matter of choice) and I am pulling a continous 22.5 - 22.8 or so, even with AC on. (this is 90/10 city driving..)
I would SERIOUSLY consider doing a UIM/LIM rebuild, and checking/changing your plugs, wires, fuel filter, air filter, etc.
Ray