Hi folks just read this chatline and signed up for my 20 cents worth..
Just pulled the cluster from my Australian compliance 1989 560SEC - it was reading 100kph at a real roadspeed of 90kph (satnav check) - no wonder everyone was passing me on the freeway. The cluster has never been changed from new. Replacing the 2 capacitors made only a small difference. I've read a few blogs saying you cannot adjust the speed reading of these Mercedes digital speedometers.
I'm a bit of an electronics nut, so downloaded the data sheet for the drive chip (UAF2115), and found that you CAN adjust the speed reading on these instruments. The 750ohm resistor next to the chip on my PCB (see photo attached) is used to calibrate the meter reading on the dial. I removed it, extending some leads out to find the room to attach a 68ohm resister plus a 20ohm multi-turn precision trimmer. I can now adjust my speed reading by about +/- 15%.
http://www.peachparts.com/shopforum/attachment.php?attachmentid=131236&stc=1&d=1438234522
I'm not convinced this is related to the REAL fault as I still have a linearity problem in this speedo so have more work to do (plus I have to design and 3D print a replacement odometer drive pinion (mine has split).
I'll make another post once I've cleaned up everything and am happy I've got a final solution.
My apologies for a very crude post - maybe I should just stick to electronics!!