Use this calculator to help you determine the "proper" size. How you deviate from the "proper" size is completely up to your own personal reasons. I chose a larger size as my "best tire size" for my own reasons. YMMV.

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

CAMRHEE Hybrid - FOR SALE!!!!
Completed Mods: RK-Sport body kit, 18" LexusGS430 wheels 235/50ZR18 tires, TRD/SE springs, Rear Lip Spoiler, Terk XM-Direct, Smoked tail lights, Sliding Armrest, Rear Bumper Applique, Front License Bracket, Lower Grill chrome inserts, K&N, Blaupunkt THB200A sub, moved rear HSD emblem
Future Mods: fog lights, black chrome my wheels

http://www.greenhybrid.com/share/fil...fterFront4.JPG