1989 firebird formula 350. what is stock gear ration in the rear end and what gear oil should I use when I change it?

Thanks!!

 

You can either use 80w90 or synthetic 75w90. Either will work just fine.

 

There's a tech data section on the front page of the forums that list the different rear axle ratios depending on engine/transmission combination you have. This is no guarantee of what the actual ratio is in your particular car; the rear end or gears might have been changed at some point.

https://www.thirdgen.org/1989-pontiac-firebird

You're going to have to remove the differential cover to change the gear lube. While the cover is off, you can count the pinion and ring gear teeth to determine what gear ratio you have.