That Tyre-Bible link is quite good -- apart from the fact that they don't know how to spell what they're talking about :mrgreen:
One thing that's glossed over is offset, though, and this topic comes up here over and over. Another discussion and depiction of that is on the TireRack site, here.
Finally, I'm really glad to see on the Bible page the discussion of contact patch. It doesn't get into the thin-snow-tire vs fat-track-tire discussion, but it gives a solid background that some people just don't think of. HPH
Check this one out, I use it all the time and find its even better than the Miata one. This calculator allows you to input not only the tire variables but wheel variables as well such as wheel size, offset, width. Then it spits all all the good info such as circumference, rev's per mile and even how far off the speedo will be @ 60mph.