Yes tomtom is 100% spot on as its done by gps. My tomtom reads 78mph when the vans speedo shows 80 so not far out at all. Whereas i know my mates Audi A3 used to read 70 on the tomtom with the speedo showing 79-80! same with his Passat come to think of it. Most other new-ish cars/vans i know are only out by about 2 or 3 at the most, so maybe its a german thing if your bmw is a fair bit out too.