My '81 300TD has an electronic speedo/odo and was a snap to fix!
I took it out and examined the gears. (which were VERY visible!)
The small gear and the first main (larger)gear had about as many teeth on them as a 90 year old Tennesee woman (no offense to any 90 year old Tennesee women).
I went to the local auto salvage yard and purchased a new entire instrument cluster for $40...that included the housing and all gauges. Removed the gears from the junk yard one, and swapped out the bad gears for good. First I made sure that no extra teeth were floating around! I could have just swapped out the entire assembly, but call me a purist...
Now it works like new! Best $40 I have ever spent.
