Really good tire size calculator at this website.
http://www.miata.net/garage/tirecalc.html
Gives you a visual of how the tires are sized differently, and a percentage of how far off your tires are. Yours are only 2%, so not that big of a drifference. You'll actually be going 2% faster than what your speedometer says, but thats such a small difference it will never matter.