Has anyone noticed that they don't match? When my speedo says 50, my nav says 48. When the speedo says 80, the nav says 76. Now wonder everyone's passing me when I thought I was doing 80

 

I don't have navigation, but I've always heard that BMW speedos generally overestimate actual speed by a couple or few miles per hour, perhaps for safety reasons. Not sure if this is true or not. Maybe someone else knows. Therefore, it could be that your nav speed is the more accurate reading.