I found this online calculator very useful while trying to determine the best match of wheel size, tire size, and final drive ratio for my S2000.
Most of the info you will need should be listed in the factory service manual for your vehicle or transmission.
http://www.turnzero.com/technical_resources.php?resource=gear_calculator
Here is a link to a good tire size calculator as well
http://www.miata.net/garage/tirecalc.html
:cool:
