Tire size calculator

I've seen a lot of posts on the subject of tires. This is a great site, will tell you the difference between any size of tire and your stock size. Tells you the difference in speedometer reading too.

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

Nice thing about that is if you have a convertable, early Swift, or a 95 and up Metro/Swift, it's already loaded for your tire diameter. Miata's came with 185/60-14 which is the same diameter as the 155-13's on the newer Metro. No worries if you have an early Metro, it takes whatever tire size you feed it.