My 84 pontiac has no rpo codes that i can find under the center console lid, as it looks like it has been put on from another firebird. so i dont know what gear ratio it is, but i do know its not posi and thats what i want to change. What size is my rear end (I.E. 9bolt, 10 bolt; 8.20, 8.50)??? also what year cars/f bodies came with posi that i can switch with and whats the rpo code for posi?

 

It is pretty easy to determine if you have a 9 bolt or a 10 bolt. I hope you can figure it out. To accuratly determine what ratio you have you will have to remove either 9 or 10 bolts (hint) and count the teeth on the gears. RPO codes only tell you what the car came with and won't reflect any previous gear changes. A few searches on ech will let you know what you can and can't do with these rears.

 

count the cover bolts, if you have 10 pull the axles and cound splines. any carrier from similar rear would swap in. g80 but that doesn't mean much other than what it left factory with. if you want to know what you have pull the cover and look.

 

whats the dufference between a 8.2 or an 8.5 rear? and how do i tell

 

.3 of an inch.

Not that you need to worry about that yours is a 7.5" as all third gen ten bolts are. There was a minor improvement to 7 5/8" in the 28 spline axle versions however. But never any 8.2' or 8.5's were offered in stock trim. there was a dana offered up as an over the counter replacement for problematic cars and to some of the Players and Firehawk series racers but they are VERY scarce.