I was recently gifted a 87 z28 says iron but no rpo codes anywhere. Today I wanted to check to see if the rear is better than mine so I didn’t a little testing. It’s a 9 bolt and both tires in the air spin same direction. However when I check the gear ratio spinning tire watching the drive shaft yoke ( no drive shaft ) drivers tire spun the yoke 3.25 turns and passenger tire spun yoke 2.5 times. So I’m puzzled is this the 3.27 rear or 2.77 rear. Vin decoding says it’s a lb9 and it’s an automatic. I just want to make sure before I pull my one leg 3.08 rear off my 85 that I’m actually upgrading and not hurting performance. Can a posi give two different rotations for each tire? I’ve never tried both tires in the past so I have nothing to go on.

 

It's probably a 2:73
Take off the cover and count the teeth.
Divide the pinion gear into the ring gear.