Some time ago there was a discussion on this board about where the speedometer gets it signal. I have the answer from a BMW master technician - this should put this open issue "
to bed". I will post it below.
The speed signal is picked up by the ABS/ASC/DSC control unit from a rear wheel sensor. It is transmitted to the instrument cluster via the IBus, which in turn, passes it along to any other control units that need it via the KBus.
The unfortunate part about this system is that there is no longer anywhere to tie in a rally computer, or any other (non BMW) accessory that requires vehicle speed.