aMaff said:To see theoretical size differences:
http://www.miata.net/garage/tirecalc.html
I say theoretical because tire sizes are a lot like shoe sizes. They vary a lot between manufacturers and models. Not all 205s are gonna be the exact (or even close) same width. With the 42 offset, you're gonna be pretty close. I have 215-45 on 17x7 +42 and rub a little under full compression in the rear. I'll probably roll the rear fenders in the near future.