Can anyone tell me what rear end I have on my 86 Z28? is it posi? whats the ratio??
no way to tell without any info on what the car was origionally, engine trans ect. and there is no way to tell what rear has found its was intot he car over the last 25 years.
best way to go about it is to pop the diff cover off, im sure it needs a oil change anyway, and see what in it, its pretty easy to see if it has a posi, and cound the gear teeth to find the ratio. if its an origional auto car, its either 2:73 or 3:23.
or you can put the back of the car on jack stands and spin a tire. If the opposite one spins the opposite direction, its an open rear, if it doesnt spin, its worn out or broken, and if htey both sping the same direction, its posi. Then put a mark on the driveshaft and rotate a tire a bunch of times, keeping track of how many times the wheel revolves as well as the driveshaft, divide and you have the ratio.
my guess is open 2:73
best way to go about it is to pop the diff cover off, im sure it needs a oil change anyway, and see what in it, its pretty easy to see if it has a posi, and cound the gear teeth to find the ratio. if its an origional auto car, its either 2:73 or 3:23.
or you can put the back of the car on jack stands and spin a tire. If the opposite one spins the opposite direction, its an open rear, if it doesnt spin, its worn out or broken, and if htey both sping the same direction, its posi. Then put a mark on the driveshaft and rotate a tire a bunch of times, keeping track of how many times the wheel revolves as well as the driveshaft, divide and you have the ratio.
my guess is open 2:73
AlkyIROC
Moderator
close
May 15, 2026
Without knowing exact model of car, transmission and engine, the closest answer is that you can get is a 7.5" 10 bolt with 26 spline axles. As mentioned above, if it's still original, more information can tell what came with the car. Anything could have been changed since it was new. The only way to know exactly is to pull the cover off and have a look. Chances are the oil needs to be changed anyway.
It was all original, 305TPI 700r4, Everything is stock
JamesC
Moderator
close
Apr 15, 2026
You should find some helpful into in the following link:
https://www.thirdgen.org/forums/tran...-10-bolts.html
JamesC
https://www.thirdgen.org/forums/tran...-10-bolts.html
JamesC
AlkyIROC
Moderator
close
May 15, 2026
86 Z28 with a 305 TPI and A4 got 2.73 gears in the diff. If it has the G92 RPO code, it will have 3.23 gears. If it has the G80 RPO code, it has posi.
If you don't have an RPO sheet, you'll have to pull the cover off the diff and look inside.
If you don't have an RPO sheet, you'll have to pull the cover off the diff and look inside.
Quote:
If you don't have an RPO sheet, you'll have to pull the cover off the diff and look inside.
whts the RPO sheet???Originally Posted by AlkyIROC
86 Z28 with a 305 TPI and A4 got 2.73 gears in the diff. If it has the G92 RPO code, it will have 3.23 gears. If it has the G80 RPO code, it has posi.If you don't have an RPO sheet, you'll have to pull the cover off the diff and look inside.
AlkyIROC
Moderator
close
May 15, 2026
It's a decal normally inside the center console glove box listing all the RPO options for the car. If the decal is gone or too faded and worn out to read, you may be able to go to the dealer with the VIN number and get a line set ticket of all the options.