Tire Size Calculator

This site has a great little calulator that saves having to do long division. I used it to figure out the difference in circumference of the 31 inch tires I was currently running, to the 33 inch tires I want to run. Then I know what sort of gear ratio I need to go with to keep my stock end ratios.

Here's what I did:
31 inch tire - circumference of 97.39 inches
33 inch tire - circumference of 103.67 inches
Miata Tire Calculator tells me there is a 6% difference which means if I kept my stock gears, the speedometer would read 100 km/h but I would actually be travel 16 km/h.
My stock ratio is 4.1. Add the 6% increase in tire circumference and I would have to get 4.35 gears (which you can't get, by the way) to stay with a stock end ratio.

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