This is a fairly common malady on the early 140 models. MBUSA knows this and because of it offer a rebuilt exchange cluster at the dealer. The price is less than a new one but not by much. I sent my defective cluster out to be repaired and got it back quickly...and with the same speedo(correct).
The price was less than half of MBUSA and with the same warranty period. I also drove the car after filling the tank(the gas gauge is gone) the whole time it was out, only had to watch my speed more closely!

Good luck,
TobiasMB
4 MBs