there are trans tags on the 4t65s its on the back in just a little farther than the diff cover. in the vehicle its facing the rack and pinion.
you can decipher the stampings on the trans with this
http://www.gmtuners.com/pda/4T65E-ID.htm
that shows what vehicle it came out of the tq convertor size and trans id code.
if you need to find a compatible trans for a certain vehicle.
edit btw the hd diff does not effect over all gear ratio. you could use an hd diff on any trans so long as you use the correct hd cover.
the above info is wrong........
here is how to break down a trans id code.... lol they do indeed exist....
https://www.google.com/search?q=4t65...-33XSjuqrTM%3A
