Another misguided newbie thread. I have a 91 Z28 with the 350 TPI motor. I looked all over the rear, and can't find a tag. Any way of telling what I have? I ran a search, but didn't find anything.

TIA!

 

I know in the 4th gens theres RPO codes in the glove box, you can use those to find the gear ratio.

Try looking for a "GU#" and use this page http://www.c2e.info/rpocodes/MODERNGMRPO.html
Found this site with a quick search also http://www.lonestarthunder.com/rpo/

 

The axle code's on the bottom of the center console door

 

if all that fails and you don't feel like pulling the diff cover to find out, here's the trick:

chock the front tires, jack up one rear tire, put the car in neutral, mark the driveshaft, then rotate the tire two times and count how many times the driveshaft turns:

2 3/4 = 2.73
3 times = 3.08
3 1/4 = 3.23
3 1/2 = 3.42
3 3/4 = 3.73
4 times = 4.11

Not sure if these are the correct ratios used, I'm more of a truck guy, but since the s-10s and f-bodies run 7 5/8" 10 bolts, so I'm pretty sure this should be accurate.

 

Once, or twice?

I know that if its a posi (both wheels spin the same direction) you spin one revolution of the wheels.

If its an open diff (you spin one tire one way and the other tire spins the oposite direction) I thought you only spun a half a revolution of the tire. Maybe its two revolutions for the open diff. Im not sure now. Anyone else know?

All of your ratios are correct though.

 

1991 Z28 with 350TPI came stock with only the 3:23 10-bolt. Limited slip RPO G80 was a required option.

 

as long as one tire remains on the ground, it doesn't matter, unless it's a locker. do not in any way mean to be a smart *** about this, but if you don't get the correct results, try it a different way. I've always used the 2 rotations and never had any different results, but then again, my focus wasn't on a broad range of vehicles.

 

Ok..found it for you http://www.thirdgen.org/newdesign/tech/techdb.shtml

 

Thanks guys. I don't have the car with me right now, but it looks to me that I need to do some upgrading. I'm thinking 4.11's. I had 4.56's in an old firebird, but that was only good for the 1/8 mile. Thanks again!