How can I determine the rear axle ratio without removing the diff cover?

 

If you're only worried about an approximation, count how many times the driveshaft turns for one turn of the rear wheels. Generally though, if you don't care enough to take the cover off, then you don't care.

 

Assuming it's the diff in your car and the diff is still original, you can easily look it up in the third gen database.

Now if it is in your car, it's also 16 years old. Any previous owner could have changed the gears or the complete diff from another third gen in it's life. If you can find the axle code stamped on the front of the passenger side tube, that will tell you what gears were originally in the diff. You can also look up the RPO codes if they're still in the center console to see what gears were originally in the diff.

However since it's probably original and 16 years old, pull the cover, check out the gears and determine the ratio and fill the diff back up with new oil.

 

I just swapped in a disc brake rear from an 88 GTA 305 TPI 5 speed, but forgot to count the teeth before closing it up after changing the fluid and gasket.

 

Well that's a bit more helpful.

88 GTA 305 5spd
Normal gear ratio is 3.08 (10 bolt diff)
If the car came with the G92 option, it will have 3.45 (9 bolt diff)

That's assuming a previous owner never changed the ratio.

 

88 gta with :
the 5 liter tbi/auto came with 2.73 10 bolt
the 5 liter tpi/auto came 3.23 10 bolt
the 5 liter tpi/5spd came 3.45 9 bolt
the 5.7 tpi/auto was a 3.27 9 bolt

No 3.08 gears in the gta that year, just in the base firebird and transam equipted with 5.0/tbi/5spd .

 

Then the third gen database is incorrect. If that GTA has a manual tranny and a 10 bolt then it has to have 3.08 gears according to the database or it's not a GTA.

 

If it's a 9-bolt, which it should be, then it's 3.45, same as the Formula.

 

There is alot of wrong info in the data base, but who am I to say.