If you have the latest VCDS, under the applications it will have the EC15/16 mileage. Press that button and read when prompted. It will read the speed recorded by the ECU apart from what the instruments recorded. Its also adjustable so can be very easy corected ! Dont believe what dealers say, any VAG car is dead easy to clock with the right gear ! The only problem is that the right gear cost up to 25ooo euros but will do many exotic cars(Ferarri, Bentleys. Lambo's, etc.) with no problems, including post 2003 Mercs and Beemers ! You can brick the ECU pretty easy on new cars but is doable...