Here is a chart that might ado help. ( this was posted by forum member "motorat" about a year ago.
i found this:
http://www.csgnetwork.com/tiresizescalc.htmlso i made a spread sheet of sizes i might use on the c5
tire width height rim size diameter circum diff stock
stock f 90 90 18 24.378 76.586 dia cir
100 90 18 25.087 78.813 0.709 2.227
110 90 18 25.795 81.037 1.417 4.451
110 80 18 24.929 78.317 0.551 1.731
rear 110 80 18 24.929 78.317
120 90 18 26.504 83.265 1.575 4.948
120 80 18 25.559 80.559 0.63 2.242
130 80 18 26.189 82.275 1.26 3.958