I don't know about you guys but changing tire sizes can be confusing to me remembering how the numbers relate with relation to diameter. I found an online tire size comparison page where you put in the stock size your came with, and then the size of the tire you want to install and they breakdown the two tires and tell you how they compare and even give you the speedometer error. Pretty cool and free to use as you feel the need. Check it out:http://www.miata.net/garage/tirecalc.html