[Supra GTR]

Here is a site I have relied on to make sure that tires will remain within a certain O.D.

TIRE SIZE CALCULATOR

Type in your factory size and then type in the tire size you are thinking about getting. Keep in mind that there are differences among manufacturers so it's not a perfect system. But this will give you a general idea of what you are getting and how far on or off your replacement tires will be compared to factory. You should stay within 5% unless you have a specific purpose in mind, i.e. racing.

 

[dohc82]

Celicasupra.com has a link to a tire size calculator that is even easier. It shows a visual of relative tire sizes compared to your stock size. It will tell you how many rpms your new size turns compared to stock and even how much it will throw off you speedo.
Very handy.

 

[quick]

I made this a sticky.

Here's the celicasupra.com link to the miata.net tire size calculator. This is the one I use: http://www.miata.net/garage/tirecalc.html

 

[Supra GTR]

I've used that one too. They are both great to see your tire sizes.

The powerdog allows you to punch in multiple tire sizes. The miata site allows you to visually see the width as well as the diameter.