This is a link I found a few years back and I thought it might help some people out here when comparing and figuring out what size tires to upgrade to. I'm posting it here, but possibly Admin can move it to the appropriate place so that everyone can take advantage of it (maybe into FAQ or something similar so it's easy to locate)?

This is a link to a tire calculator from a Miata forum. Basically, scroll down to the bottom, enter your current (or OEM) tire size, as well as the size you're contemplating, and it'll calculate % error in rolling circumference as well as speedometer error. You can use that to then revise your tire size selection so that you can size a new wheel/tire package to keep it as close as possible to your OEM rolling circumference. That way, your instrumentation (speedo, odo) calibration isn't screwed up, and with Quattro, it may (not sure) be more important yet, so that AWD or skid control system calibrations aren't wigged out. You can use it to check any plus- or minus-sizing package that you want - just type and hit calculate. Anyhow, that's what it does. Here's the link:

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

It's helped me out in the past, and hopefully someone else here will benefit from it as well.