well she has 180XXX miles. everything is up to date as far as i know. there is one 02 sensor that isnt in right now. and im about a 50-50 driver half highway and half city. but when i was workin 80 mi one way to work everyday i got home and had to fill back up bc it was about empty all highway

 

If you have original O2 Sensors at 180k -- go change them all out... Try Densos, see:
- Camaro Home Page Camaro Project Part 3

 

im pretty sure the o2 sensors are fairly new

 

If you are not sure you should make it so you are sure. If you did not replace it or have it replaced then you should assume it needs done. New O2 sensors do not last long if you put into an engine that is not running right or burning oil or coolant also a partially plug Cat can cause a O2 sensor to fail pretty quickly. The forward O2 sensors are the gate keepers for the engine running efficiently. The one or two by the cat just monitor the cat. There are a lot of things and can go wrong that will cause bad mileage but in most cases a good working O2 sensor will set a code letting you know the computer cannot tune out the issue.
There are some things that will not set a check engine light and can kill your mileage but most of them would have other symptoms. Example: Lose of power, high idle, Rgh idle, Stumbling when accelerating or hesitation and tranny issues.