Try this link.

http://www.eunos.com/tirecalc/

You just enter the original tire/rim sizing, and the new tire/rim size. It will give you an idea on what the tires will look like size wise compared to the old set, and tell you what it will do to your speedometer readings.

P.S. I just put Toyo Proxes S/T 275/55 R18's on my VX and they fit perfectly. Remember, this is with an 18" Rim though.