1996 Infiniti I30 poor gas mileage and rough idle

I am getting poor gas mileage and rough Idle on my 268000 mile infinity. Does the Rear Cat if clogged do this. The temp readings are low at the exhaust side and the O2-12 sensor readings vary. Or is this just a bad O2 sensor.

Poor mileage is a symptom of a clogged cat as is power loss at high rpm. Rough idle, not so much.

If you car is not showing any codes (like a P0420) and your oil consumption is not greater than 1500 miles per quart, then it probably is not the cat.

Is the check engine light on? What are the error codes? Is the car warming up properly, if not, that indicates a bad thermostat and that will cause poor mileage.