275/45 is something like 10% bigger than oem in circumference...that's going to have a big effect on gearing (it's going to slow you down). I would try to stick within a couple % of stock size.

Not sure where the speedo feed comes from (front or rear)...I assume front, but if it is rear, your speedo (and odo) would be way off too.

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