forget it - don't buy another one

I have had a sticky speedo too in my 1986 240. Here is comes. I pulled the whole cluster out of the dash. Removed everything at the back of the cluster. Then finally you are able to pull out the speedo. Ones you've got the speedo in your hands look carefully for a tiny bit of paper. This annoying paper thing has some numbers. This paper is actually sticker! But it won't stick anymore after sticking for 13 years. So if you find this loose bit of paper, just take it away. Reinstall the everything and hit the highway :)

If this sticker becomes loose then it blocks the needle from freely moving around. And that's why it moves a bit when you hit on top of your dash.

You didn't find anything? Then look for a used speedo.

Hope this trick works for you too