You know, it just occurred to me that your problem may be related to the
little spiral spring in the speedometer.  It may be out of whack.
(And, no, don't 'whack' it back into shape ;^)

Speaking from experience here...the little plastic cog in the odo on my
78 Scirocco once  went out, and when I fixed that and put the whole
shebang (now there's a technical word!) back together, my grubby little
fingers must have accidentally bent that spring because the speedo was
off.  Man, what a pain in the ar$e to set that right!

Joachim
--
I'm not an engineer, but I play one at work.