Wheel offset came up in another post and i went looking and came up with this site.

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

Compares new wheels to existing

Great for when you are buying those new mags :thumb

Here's another couple:

Tyre size calculator: http://www.miata.net/garage/tirecalc.html

A whole bunch of useful wheel and tyre calculators

http://www.customspeedparts.de/content/en/technic/felgentechnik.php