I couldn't find the link you folks use, but I found this and decided to share

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

Nice and simple.

The one we use is number 15 in the Techinfo -- Tech Know-How. Here is the link. It looks like the one you posted just compares tire sizes the user enters where this one accually caclulates the new tire size within a caclulated difference.

http://www.c5-corvette.com/tirecalc.htm

Read the instructions at the bottem of the page they help a lot. Especially the one where you can select TIre B'* wheel size so it only gives you a certain wheel size other wise it will give you a list from 12" to 22".

With my 235-55-16'*, Corvette'* log say I am speeing by 1.73%. Not bad.