Easiest way to do it if you have the 3rd out is to count the teeth on the ring and divide it by the number of teeth on the pinion.
Chances are you either have
41/10=4.10
41/9=4.56
or if you're one of the lucky few
39/8=4.88
Go here for more information than you could have ever wanted to know...
http://www.4crawler.com/4x4/4R_TechI...#Differentials