Wheels / Tires Fitment Calculator
 

I discovered the following link which allows you to compare your existing wheels and tires set-up to whatever new set-up you are contemplating. It provides the calculations for the differences in wheel-well space, speedometer error, etc.

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

The best one IMO.

I agree. I used this one after trying several others to spec the winter wheels for our S5 and Explo Sport, and to spec my new 20" wheels for our XKR. It's the best one I've found.

I like the fact that not only does it provide a comparison in tire width and diameter, it also provides the comparisons in wheel well,fender and suspension clearance, as well as in speedometer readings and stance.