Check this out, it gives a diagram for almost all the specs you could look for in a wheel tire combo: Tire size, rim size, and offset! http://www.rims-n-tires.com/rt_specs.jsp?postId=906
http://www.miata.net/garage/tirecalc.html this works wonders as well...you can actually see what the relative difference between your stock wheel and a different size will do in terms of your speedometer...
Yeah, I found the miata one first, but I just thought it was really cool to include offset and all those measurements. Granted, I don't actually NEED to know all that stuff, its a little over the top for most of us.