When going from one size rim to another is it as simple as switching from a 16" to a 17" or is there something else that needs to be done. If clearance is alright are there any inherent problems with going to a larger size?
or you could just get the correct sized wheels it should be fine... if you go up a size in rim you have to go down a series or half series in tire. Heres the link to a website that should allow you to input the stock/tire combination size to a plus size and how much the speedometer will be off it any! http://www.miata.net/garage/tirecalc.html
i don't like that miata tire calc. i use this one instead...you can even have it search for tire sizes the same overall diameter or close to your stock size and it gives a long list of MPH and ODO differences between two sizes as well as all of the dimensional differences.