I'm trying figure out the diameter of a ring gear. Do you measure it at the very outer edge teeth. When I do I come up with 8-3/4" I hoping that is what I have.

just WAG ing. it'd either have to the the OD (likely) or the ID (least likely I'd think) or C to C (maybe) which'd be between ~ the (1) and the (8) on your tape (7"). I'd do all three & see if one of em is 7&1/4/8&1/4/9&1/4/9&3/4. Here's a BTT for ya

Actually, I found the part number and date code on the ring gear. It matched up with one in my 70-71 parts book and it is a 8-3/4" ring gear.

So now you have to figure which case its for ... 489, 742 or 741. You do have the pinion, right ???

I do have the matching pinion. It is a 1-3/4" so the gear set is for the 742 case

It's always the OD of the gear. The gear outside the contact patch isn't that important, so they sometimes are a little smaller OD than the name would suggest.

When hypoid gearsets are made, they get put into a machining setup and the pinion and ring are assembled and run with lapping compound for a brief time. That makes them a set, and unless you're building a rear to get out of the Yukon or a place like that, sets are never broken up. Pinions aren't swapped out without changing the ring gear to the matching one. I put this last paragraph in for the newbies out there.

R.