There are other sites around that offer gadgets for playing with wheel and tire options but this one makes it simple and visual: http://www.rimsntires.com/specspro.jsp

Agree.  I've been using that site for years.  It's a great tool for checking various sizes.

Beware in the first 10 seconds that I was using it, it changed tire and rim widths on me. This calculator will not allow you to stretch a tire. If I put my rim and tire specs in there it'll change one or the other, changing backspacing.