i wanna find out what kind and size reaer end i have in my 91 firebird. it has a 305 and had an auto. there was a number on the bottom of it its 22522676. i really want to find out what the size is, i cant seem to find it from any research. from wht i can find its probably a gm 10 bolt open with 2.73s, though am not certain, but what i want to know is the size 8in, 7.5in, 8.5in ... ? any help would be appreciated

 

Count the rear end cover bolts.
9 bolt heads = A 9-bolt.
10 bolt heads = A 10-bolt.

Stock is the 7.5". You can jack the rear of the car off the car (use jack stands not just the jack), put the tranny in Neutral, then rotate the tire 1 full revolution & count the # of turns the driveshaft makes. 3 tire rotations to 1 driveshaft rotation is equal to a 3.08 gear. 2 3/4 tire rotations is equal to a 2.73 gear.

Those are your 2 most likely gears, for your combination.

As for Open vs Posi....If Open, only the Passenger side will turn or if both turn, the Drivers side can be very easily stopped & the tires will rotate in opposite direction. A Posi will make them rotate in the same direction.

 

If the rear is OE, it's a 10-Bolt, so here's the FAQ:

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

 

According to the TGO tech data, the only option for a 91 Firebird with a 305 and an auto is the 2.73 gears. Your diff will be a 7.5/7.625" ring gear size and it will use 28 spline axles.

That's all assuming it's still original.

 

thanks for the help guys