Gear ratio identification

[84SierraClassic]

Hey guys. I am trying to identify what my gear ratio is in my rear axle, without having to tear it apart. I have tried looking it up with the RPO code, but all I can find is GQ1=standard axle ratio. Now, looking at RPO codes for the past 2 days I have found that gear ratios have codes GT*, or GU*. Now, I only have 1 sticker with RPO codes on the vehicle, on the left inner fender. No sticker with RPO codes in the glove box. So I am at a bit of a stand-still.

cliff notes:1984 GMC SierraClassic 1500. All stock with the only exception being the motor. Only RPO code I have to go off of is GQ1. Rear diff is GM Corporate 10-bolt.

I will get tire size info, and what RPM at what speed once I get a tach installed. There is a way to find out what gear ratio is by some formula isn't there?