I did some more research, and e-mailed some German eBay sellers to ask about the speedometers they had for sale. The information I discovered led me to believe that the speedometer reading is based on driveshaft RPM, which is dependent on the rear axle ratio, so your replacement speedometer must be from a car with the same rear axle ratio.

I found a German website with a table showing the different rear axle ratios that were offered in the 190. There were many more than I thought there would be. I translated the table to english and a copy is attached... If anyone else is looking for the information on which differential was in which car, you can refer to this table.

Attached Thumbnails

 

__________________

1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD