I JUST ORDERED SOME RACING HEART TYPE C5 WHEELS IN 18X8 IN THE FRONT AND 18X9 IN THE REAR I WAS WONDERING WHAT SIZE OF TIRE TO PUT IN THE REAR?? ANY THOUGHTS???

 

255/35/18 is very close to OEM circumferance.

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
205/55-16 4.4in 12.4in 24.9in 78.2in 811 0.0%
255/35-18 3.5in 12.5in 25.0in 78.6in 806 0.6%

Here's a tire size calculator for you -

www.miata.net/garage/tirecalc.html