just by looking at a ring gear, how can you tell what gear it is? (3:55, 3:73, ect)
Thanks
Kev

Most ring gears have the ratio stamped on the outer circumference somewhere. If not all you need to do is count the ring gear teeth and the pinion gear teeth and divide the pinion count into the ring gear count.

IE: 41/11 = 3.73

Here is what you will be looking for. From left to right the numbers are: part number, tooth count (the numbers to put into the formula charbilly posted) manufacture, date and the last number is probably the shift that machined the gears. This is a set of 3.08 gears from an early build 64 El Camino 8.2" rear.
http://i80.photobucket.com/albums/j199/64elcamino/ringgearID.jpg