If you want the right size rims and tire combo, use the following tire calc. It will help you decided what size is best so the car can run as if it were running on stock wheels.

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

for example if you your car OEM's are 195/65/15 then 225/45/17 is the best so the speedometer stays balanced, you will be doing 60mph when the speedo reads 60mph, other combos will make your speedo read 60mph but u might be going slower or faster than that.