im currently running 185/65R15 on my car currently with stock alloy rims.

I was thinking of getting some 17" rims what size would they need to be to stay the same circumfrence? (i dont want to screw my speedometer up with a different size)

Hopefully i can do this for summer and stick some snow tires on my current rims for winter.

thanks in advance for your help!

Check here....

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

Simon.