Tom;

Factory diff ratio for Automatics was: 4.30, shifters had: 4.10.
Typically when the conversion to a manual was done the speedo-gear in the tail of the trans (which is supposed to be matched to the diff ratio) was left in place, causing the speedo to be generous by the ratio 4.30/4.10.

Kinda Like this:

(from the article: http://www.intelab.com/swem/j5rhistory.htm)

If the tag is no longer in place, and you want to verify your diff ratio using a method which is bit cleaner than pulling the cover and dealing with diff oil etc., (I call it the proof of the pudding method) jack up one rear corner, leaving the other one on the ground and locked from turning, then, with trans in neutral, turn the driveshaft 10 turns while having a helper count the number of times the wheel turns. The ratio of number of turns will work out to either 4.30 or 4.10.

If you can't turn the DS when only one corner is in the air, consider yourself very fortunate...you have a limited slip diff!!! Ratio verification procedure is the same, just requires BOTH rear corners to be off the ground!

Cheers