There are actually three different types of speedo/odo. Mechanucall driven, electronically driven and digital.
The first and second types have a single input and that is converted to two different displays (actually digital does this also but differently.
The mechanical uses a cable which spins a magnetic speed disc inside the speed cup/drum that is attached to the needle. The odo like any straight counter is just geared off of it.
The electronic takes a pulsed signal and separately creats the speed and odo displays. The odo display is rotated from a motor through gears.
The digital gets its input usually from the ABS via CAN and calculates and displays speed and odo readings separately.
Bosch Master, ASE Master, L1
33 years MB technician