I'm looking into getting a new set of tires this spring for the Miata and was wondering what would be a good set to use.  I'll be using them for street and  occasional autocross events.  I'll be limited on size due to not many wide tires run in 15inch and I already have a lightweight set of 15's so i'm sticking with them.  Also, what size tires do you guys suggest me running?  The Miata has some power so I figured I'd try running 225's in the rear.  Tires on it now are 205/50's.   
Edit:  Wheel size i'm running is 15x7   
Any suggestions are appreciated.   
Thanks, 
Paul