I posted this in the TPI Board, but only got one response. Figured the topic of 'gas mileage' was beaten to death, but i just want to know where i may have lost mine. Before my Fuel Pump replacement i got around 205 - 211 miles on a tank of gas. Now i get maybe 75 - 116 on a tank of gas. The only thing i have done lately is stick a pipe through the cat and replace the O2. The O2 btw was black and my fuel pressure is at 46 PSI (i don't have a afpr either ). I do hear a...kinda pop, or boom type sound at random intervals in my exhaust, but i'm unable to hear it under the hood. Like i said, i just wanted to know where to big jump down in mileage came from. oh and my plenum is the only thing ported and the runners are port matched, the manifold wasn't touched...yet

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89 IROC-Z 350 TPI

-Flowmaster Catback
-Performance Resource Chip
-700R4 (Rebuilt) Too much done to actually list
-K&N Airfilters
-Ported Plenum
-2.77 Gears (not much to brag about but eh, its there)
-MSD 8.5 mm plug wires
-Gutted cat
-!AIR

 

There's a lot of things to consider. First of all, where are you driving? Your mpg on the highway will be much better than in the city. Also, how are you driving (leadfoot)?
Be sure to check your tire pressure. Underinflated tires can cause you to lose a lot of gas mileage.
For as much of a loss as you are experiencing, I'd say you're running rich. I know you just replaced the O2 sensor, but remember that it can be 'tricked' into causing a rich condition. For example, if you had a cylinder not sparking, none of the gas or air in that cylinder would burn. Instead it would go out the exhaust to the O2 sensor. The sensor would see that the oxygen content was too high, and the computer would assume that the car was running lean. The computer would dump in more fuel, but the O2 sensor would keep reporting a supposed lean condition. Then the computer just dumps in more gas. The same problem can be caused by an exhaust leak which occurs before the sensor. There could be other things that would cause this sort of problem, but I'd check the plugs, wires, cap and rotor first.