Want to plus-size, change offset, change rim size... this will give you the EXACT specs of comparing your current rim/tire combo with your proposed rim/tire combo.

You enter in the rim and tire specs for rim diameter, width, offset, as well as tire size and every measurement is shown.

http://www.rims-n-tires.com/rt_specs.jsp

Awesome site!!! The only that would make it better would be a visualizer to allow you to see the rims on the car.