I've had this Garmin GPS for a while and when going 30km/h (ish) the GPS tends to show values between 35-40km/h. The GPS also indicates movement when stationary. Could the device be faulty or is this kind of behaviour to be considered as normal.
No that doesn't sound right mate. GPS can lag a wee bit, so could still display some speed just after you stopped, but should otherwise be accurate.
Thank you for your answer. I contacted my local stockist and they will send it back to Garmin as faulty.