If you type miata tire calc in Google, you can try out a bunch of size
combinations to see what will fit.  Wheel offset is important. I run 18's
(AMG - Mercedes 18x8 on the front and 18x9 on the back) on my bus,
obviously with the correct size tires. Don't know how knowledgeable you are
on tires but the 1st number is generally the width of the tire, the next is
the %age of the width that is the sidewall height and the last is the
actually wheel size.

Nij