What I mean is this: both the W116 and W126 get their info from the
transmission.  AFAIK, one (W116) is cable driven so have some kind of
gear mechanism on the transmission the speedo cable goes to.  And, the
W126 has probably some Hall-based thingie which then sends a voltage
pulse to the speedometer wiring. I am calling both the electronics and
mechanics pickup sending unit because they both do the same -- measure
shaft rotation -- in a different manner. I would guess they both are in
the tail of the transmission so they are after all the gears and are not
affected by them; the only gear that can cause them to be off is the
differential (and the tire size).

So, if my terminology is not accurate, I do apologize; I do not claim I
know that much about cars. But, I hope now we are talking the same
language. =)

Now, why I am so curious about the different sending units?  You see, I
dropped a engine + tranny off a 83 300SD in my 75 300D.  As you know,
the W115 has mechanically driven speedo while the W126 has electrically
driven. I am trying to decide if I am going to use the W126 speedo (just
a matter of redoing the cluster) or use the stock W115 speedo and
somehow change whatever I need to in the tranny to send a signal to the
mechanical speedometer. To do that I need to know what I am looking for. =)