As I go through the myriad of web pages concerning tire sizes, dimensions, and availability, I found this calculator that helps to determine critical dimensions. This really helps when you are trying to determine what will fit in your available space especially if you are trying to avoid buying a new axle.
http://www.discounttire.com/dtcs/infoTireMath.dos
Note the disclaimer that this calculator is based on formulas and that actual tires of different manufacturers may vary in dimensions. This should still put you in the ball park.
Larry S.