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.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician