Speedo problems are very difficult to diagnose. Everything from tire size to the drive gear in the tranny can impact operation. However, if I were you I would replace the cable first as it is significantly cheaper than rebuilding the speedo. And when I did, I'd have a look at the drive gear to make sure it's OK. It isn't hard to do, but you are working in a confined space and it will take some time. If you search here you'll find the exact procedure outlined. If that doesn't solve your problem, you'll have to have your speedo rebuilt. Someone here does them, again, a search will tell who. Good luck.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick