Tires
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#2
Plug the numbers in the windows after you get to the site in the link attached. Feel free to play with the numbers to find a tire size that is a better match. You are best to find a tire/wheel combo that matches the OE size as best as possible. The factory tire size is best, assuming that all other parameters are stock still also. Those tires will be smaller, and your speedo will read faster than you are truly going.
http://www.miata.net/garage/tirecalc.html
http://www.miata.net/garage/tirecalc.html