Originally Posted by gpb

I'm no expert, but I do know that tire designations are x/yRz where x = width in millimeters, y is ratio of sidewall height to width (x), and z is rim diameter.

Thus a 255/75R17 is 255mm wide (or 10" when you divide 255 by 25.4 millimeters/inch). Sidewall is 75% of 10", so 7.5". Rim is 17", so total height would be 17 + 7.5 + 7.5 = 32".

To figure out a 12" wide 33" tire just go in reverse. 12 x 25.4 is about 305mm, and to get 33" on a 17" rim you need 8" sidewalls; 8/12 = 67%. To meet your desired dimensions you'd want to find something like a 305/65R17 or 305/70R17.

Pretty elementary once you know what the tire codes mean.