I'm just about finished with my LT1/T56 swap and I have a feeling I'll need posi soon A friend just swapped in a 4.10 rearend into his '91 Z, and he said I could have his old rearend for mine. We looked up his RPO codes and I think it said 3.22, or something near that.
My question is, would I see a noticable difference in performance, and MPG if I were to switch rearends? I think the only real reason I want his is because his has POSI..mine doesn't...

 

I just did the swap from 2.73 to 3.23. If you are looking for a BIG gain, look else where. I did feel a little difference in performance, but the biggest reason I did the swap was for the Posi and disc brakes. As far a gas mileage is concerned, I couldn't help you. I haven't driven it long enough to tell.

 

2.73 and T56 is a really BAD combination. It'll bog off the line like you wouldn't believe (ask me how I know.) If the 3.23 rear is cheap do it. Also, the 3-series carrier will make it cheaper to swap to 3.73's in the future. IMHO 3.73 is the perfect gearing for a 6-speed car.

MPG probably won't change much. 2.73's make 6th useless at anything under 80, so any mileage you lose to higher RPMs you gain right back with the extra useable gear. My mileage didn't change much going from 2.73 to 3.70, so I know yours won't suffer.