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.
Current: '91 300TE 4MATIC 317k and climbing...
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)
"Never sweat the petty things...
and never pet the sweaty things."