I was looking for a tire size calculator and the only one I could find before was the one at that miata site that uses flash. I found this one just now and I just used it to decide on my new tires. Let me know what you think.

http://www.1010tires.com/TireSizeCalculator.asp

The Miata calculator uses Java and I like it better. The Miata calculator doesn't take so long to full out and gives you a nice visual + the numbers. Just download Java and you'll be set to use the Miata calculator.

I highly do not suggest you go off of a generic tire sizing chart like that. Reason Why?- You can take two tires from two DIFFERENT manufacturers BOTH 245/50-16 and have two radically different section widths and tread widths. Its just like jeans or tennis shoes, size will vary for manufaturer to manufacturer. You need to just look up actual documented facts on a websdite like Tireracks specs- However, they do sometimes have misprints but are mostly pretty accurate.