Hi Jughead,
Thanks for the answer. The rear drive ratio is correct, (I have checked the numbers engraved at the bottom of the housing). I can not check the size of slots on the pickup ring, as the bike is not with me at the moment.
The speedo sensor was covered with steel dust, but cleaning it and changing the rear drive oil did not change a thing.
However, as the bike indicates speed correctly, but counts the distance at a 1,625 ratio, I do not see how the wrong pickup ring can give correct speed signal, and incorrect distance signal? In my opinion, if the pickup ring is wrong, it would influence the speedometer as well.
I think that some of the previous owners either "played" with the total mileage and messed something up, or the bike was damaged, and a new (second hand) instrument cluster was improperly fitted.