Dimensions from the tire calculator chart at http://www.miata.net/garage/tirecalc.html

245/60/18 285/55/18
sidewall 5.8" 6.2"
Radius 14.8" 15.2"
Diameter 29.6" 30.3"
Circumference 92.9" 95.3"
Revs/mile 682 665