A very good tire size calculator is at the site below. It gives a graphic representation of a new tire vs the original one you are comparing it to. At the bottom of the page, you can compare tires of different sizes and it will tell you how much each differs from the original. It gives all the tire dimensions, revs/mile, and % change from the original. A very helpful site.
http://www.miata.net/garage/tirecalc.html