The speedo has a built in error so that when it says you are doing 45, you do not get a ticket for really doing 50. You are doing 40 instead. The speed you are doing is not as important than the idea that you are really doing less than the speedo say, and you are doing it for the reason I gave.

Using the On Board Computer, reset the speed while you have the cruise control set. The initial recalculation is the actual speed as calculated by the rotation of the tires, not the speedometer. The speed in the OBC is an average over time, but if the Cruise is activated then the initial speed displayed at the moment of Recalculate is the actual speed. It's still an average over time, but the time is short so the display is actual. You don't need to set the Cruise, but if the Cruise is set then the displayed speed will be constant until the speed changes and the Cruise tries to keep this from happening.

The speedo has no correction that I am aware of. You can cure some of the error by using larger tires. I've been on BMW forums for 10 years, and the only thing I know of to change the speedo is different tires.

Everybody has the same problem as you, to some degree or another. Back in the olden days of cable-driven speedos, there was a calibration process that involved a dyno. If there is a similar calibration for the electronic speedos that are used today, I do not know about them.