young noob

hey guys, my gas mileage has been **** lately, and i have done all the little tune-ups and ****, and i always thought about replacing my o2 sensors, but they are pricey, finally i threw a code and this morning i went to autozone and wouldn't you know it, it was my pre-cat o2 sensor, im going to replace both with a hook-up from a friend so i don't have to pay the ungodly amount, my question is, i was getting about 26 mpg city with my 96 cx hatch, what have you guys experienced with better gas mileage after replacing your o2 sensors....thanks