Just recently that my GPS is showing around 5% less than the speedo does. Another owner reports even 10% what is a lot.

I really wonder if worn tires may affect the reading that much. Note, these are 20" wheels and worn tires travel less distance on each revolution. And the car has no idea about that so there is no way to compensate. But we have a SAT/NAV installed so it could use these data t oadjust. Hmm, we all know that the cars aren't that smart and the different control units are not interconnected. For sure the ECU has no clue if a SAT/NAV is on board or not.