Okay guys, I know I've asked alot of questions about this. But maybe this is the last one. I have a 2.73 gear ratio in my Chevelle right now. I need to put a 3.73 in. There are all these options, i.e 8.5", etc. Is that the measurement across the differential?? Mine looks like it's 10-11" wide.

Basically, can I change my ratio without having to replace the entire unit, and how do I do it?

Thanks again!!

If you have a '72, with a 10 bolt, there is a 99% chance you have a 8.2 rear. To run a lower gear, you have to get a 3.08-up chunk, and a new gear. I don't think they offer a 3.73 for an 8.2 Chevy, so your next choice is a 4.11, or go up to a 3.55 ratio.

How can I tell if I have an 8.2" or an 8.5"???

8.2" vs 8.5" is the size of the ring gear, not an outside case measurement.
Look at the pics in this link;
10 bolt ID (http://www.oldsmobility.com/10bolt-tech.htm)

8.2" vs 8.5" is the size of the ring gear, not an outside case measurement.
Look at the pics in this link;
10 bolt ID (http://www.oldsmobility.com/10bolt-tech.htm)

That was an excellent link Epistuff!!! That was exactly what I was looking for!