This 11 minute video is worth every minute...
http://www.youtube.com/watch?list=PL...&v=UOMNFb8kOIU
If you want to save some time start a 6:55
Watch the very end.....
The approach the shop takes is super detailed and backed with a very interesting method of measurement. Tire and wheel size is based on the car's dimensions and not "theory".
Here's a PDF for further information
http://www.ammonyc.com/wp-content/up...StepbyStep.pdf
Hope this helps a bit....