As far as tire size, play with this site to get as close as possible to the stock tire diameter: http://www.miata.net/garage/tirecalc.html
The size you are looking to convert to will result in a 4.9% increase in speed compared to what is indicated. So if you would be traveling about 3 mph faster compared to your stock sized tire at a given speed (say 63 mph as opposed to 60 if the spedometer is saying 60 mph).
Most vehicles are actually traveling slower that what the speedometer indicates in stock configuration anyway. My CR-V is about 3 mph slower that what is indicated with it's stock tires (205/70R15) but has been corrected with the 215/75R15s that I run now. So I'm actually going the speed my spedometer says I am now.
A tire size change will also affect your odometer reading, mine is about 10 miles per 200 miles less now. Not a big difference but just so you are aware.