i have a zr2 with 33s and i just recently drove down to sc and im getting about 12 mpg on the highway....this doesnt seem right to me im throwing a code for a post cat o2 sensor but that was recently replaced....before i left for the trip i did plugs, wires, cap, rotar, and new ignition coil...i figured id atleast get 15

 

Well it's hard to say with the 4.3. gas mileage isn't it's strong point. Adding 33s into the mix doesn't help. That's probably where you're losing a mile or 2. More rolling resistance = worse mileage.
The O2 sensor would definitely cause bad mileage, so I'd make sure they are all in good working order. Are you throwing any codes?

 

well, i'm lucky to get 15-16 average, and the only thing i've got aside from 8mm plug wires, and an MSD coil, is a TBS, i've been trying to find something to help