Ever wonder how another wheel/tire size compares to what you're running now? I've found this calculator gets the job done:

http://www.discounttire.com/dtcs/infoTireMath.do

In my case it showed that 245/75-16; 265/60-18, 245/50-20 and 275/45-20 were all nearly the same on key dimensions.

BTW, 6 x 5.5 wheels with a standard offset, very common on Chevy/GMC truck applications, are easily found replacements on a VX.