I may get radical and change out my wheels to 18" if that's what it takes

The speedo may be designed around 18" wheels. Then anyone with 16" or 17" wheels get the benefit of the Porsche factory "safety-margin" feature

(sarcasm) You'd think with 3 different wheel size options there would be and adjustment for all three. Maybe there is???????? I could only hope. However I love my 17" wheels with the cool lookin Pzeros.