Hello Rainer;
I have an 87GL, the little number under the VDO is 500.  This is a Canada
market vehicle, in kilometers.  I have 205/14 tires on it, and I know the
speedo is accurate, as I have had it tested against radar (don't ask).
In my GL the speedo cable (inner) plugs into a square hole punched in the
left front outer wheel bearing dust cover, from there the cable goes
directly into the back of the speedometer, there are no gear reductions
along the way.
I have seen some USA market VWs with a O2 idiot light in the dash, which
have a re-settable device in-line with the speedo cable.  The function of
which is to remind the operator to check his O2 sensor (or have it checked)
at 60,000 mile intervals. I don't think there is a gear reduction in this
device, just a counter.  My Bentley manual doesn't mention any gear
reduction anywhere along the speedo cable's run.
I've wondered where the cable  takes it's signal on a synchro.  I doubt
that it would be the same as the 2-wheel drive versions.  Larry Carter