What rear gears did the 86 Z-28 come with?it is the 305 tpi with 5-speed, originally auto

 

i believe they came with a 9-bolt 3.45 gear.

if its a 10-bolt it should be either a 3.23 or 3.42

 

put a piece of tape on the bottom inside of a tire, and a piece of tape on the pinion yolk going into the rear(absolute bottom of each)... now spin the wheel (without the tape, so you can see both the rear and other wheel) and make one full rev with the wheel, count how many times the rear yolk spins... it should be 3.23x or 3.42x is it's 10 bolt. 3.45 if it's a 9-bolt.

just a trick i happened to pick up from my father