I'm really just trying to find out what max tire size I can fit on a 1st gen. OE alum. rim. I currently have 265/70 R15 tires. I would like to get some nice off-road mud tires. Any ideas?

Try this http://www.miata.net/garage/tirecalc.html

Since your asking about max size tire - on a stock aluminum wheel:

The info may vary depending on what width tire you try to run (31x10.5 will fit with much less problems than 31x16.5 tires)....but since few people go wider than 12.5 inches:

31" tire fits 2002 Xterra, and 2000-2001 with minor trimming (plastic). Mudflaps, however, are likely not going to work with 31" or larger tires on a 200/2001.
2002+ will easily clear 31" tires with the mudflaps on. You can mount the 2002+ flaps to the 2000/01 flares by drilling 2 holes and trimming the flare to match its contour, therefore you can run 31" tires with mudflaps.
You can even clear 32" tires with the 2002+ mudflaps with a teeny bit of additional trimming
32" tire fits any Xterra with major trimming and a lift is recommended.
33" tire requires a lift, or lots of trimming and work arounds.

The best one i have found is the following


http://www.1010tires.com/TireSizeCalculator.asp