Damion gets a low-profile no-prize for bringing this automatic tire calculator to our attention. It says "miata", but it'll work with anything as long as you know the size you started out with and the size you want to go to.

Your speedometer was calibrated with the stock tires it came with. By changing the size of the tire, unless you're careful, you'll throw your speedometer off by certain amount. This calculator will tell you just how much. Very useful when you want to get wider tires but don't want to send your speedo into la-la land.