I have a 88 GMC Sierra. Does anyone know what kind of rear diff is on this year? It looks to be a 10 bolt 8.5 but I'm not entirely sure? Also, does anyone know what gears came with these trucks? How can I find out without tearing it open? Thanks.

 

should be a 3.42

 

it is a 8.5 and the best way i know to tell what gears you have is to jack the back up and put a mark on the ground and the tire line them up and mark the drive shaft to the rear houseing make sure all lines are lined up then ture the ds and count how many times you ture the ds for the tires to make one ture and that give you your gear

 

I had a 90 GMC Sierra. It had the 8.5" 10 bolt differential with 3.08 gears. You can call your local chevy/gmc dealership and give the vin to the parts guys and they should be able to look it up to tell you what is in your truck. On a side note, if your thinking about getting a limited slip differential, get a Detroit Truetrac limited slip unit. They are priced pretty low and work awesome.

 

I found out that I have 3.08 gears. I have checked into the Detroit True-trac and they have two models for my truck - one with 28 splines and the other with 30 splines. How do I know which one I need?

Also, should I change gears? What gear would be best for a built motor that I mainly only drive around town?

 

With the weight of the truck and all, I would consider something semi low. I ran a 4.10 gear in a 90 gmc sierra that I drove around town and on the freeway. It had a 700r4 transmission so it ran good around town and on the freeway.

 

Yep, the biggest gear you could get in a 10 bolt from 88-95 I think was a 3.08. 3.73s and 4.10s came in the 454ss in a 14 bolt.

I have 3.73s and if you do a lot of hwy driving, I would recommend them over the 4.10s. 3.73s is a great all around gear IMO.