Tire Size and Wheel Comparison Tool
This is the most awesome wheel/tire size comparison tool we've seen on the web! It's very easy to use, simply plug in your stock (or current) tire, wheel size, and offset on the left, and your desired tire, wheel size, and offset on the right and it will show you all of the measurements of the configurations as well as give you a hint as to whether or not your thoughts will work on your car or not. The best part is, that it will tell you the difference in tire sizes as well as offsets, which are tough to figure out.
Here's the link to the tool: http://www.rimsntires.com/specs.jsp