When I put the 16" Centra's on my car, I checked the standing height of the tires I was looking to put on and bought something near the same standing height(dia.) so the Speedo wouldn't be that much off. My 80 MPH reading now is 76 MPH by the GPS. So it reads a bit fast at 80.
BTW let me know if you are going to sell the Centra's on your car. I need a set for my 560SEL.