Online tyre calculator..

[D4rk4n63l]

Dont know if its been posted before (I did search but did not find one), but here's a good website if you want to calculate the dimension of your tyres. It will also calculate the correction factor of your speedo as a result of changing tyres/rims of different sizes...

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

[EuroDude]

repost ftw :)
http://www.ozhonda.com/forum/showthread.php?t=26144&highlight=miata

[D4rk4n63l]

my bad then... :p

[Zdster]

Yep, these have been around for quite a while, but very helpful.