Hi Harald
The obvious question is, has the instrument failed or is there a problem with the speed sensor or its wiring. You could of course simply take the speedo out and send it off for testing. According to DaveW's book however (see page 125 if you have a copy) the speed sensors are quite delicate.

When I had my speedo out of the car to take to the company that RichardV6 points too in his link, I decided to knock up a very simple oscillator to determine the relationship between the speedo reading and the speed sensor pulse frequency. This is a rather crude but simple circuit that enables you to run a rudimentary go no go speedo test on the bench. The oscillator is controlled with a potentiometer that gives you the ability to vary its frequency from about 14Hz to 166Hz and so in my case this varies the speedo reading between about 6mph and 80mph. If you or someone you know has reasonable soldering skills it would not take long to build on a piece of 0.1 inch hole spaced Vero or strip board. All the components needed are available on ebay Connections to the speedo can be achieved with 2.8mm standard female spade connectors. I have included the circuit diagram see below. If you chose to have a go it is of course at your own risk but as you can see from the photo, also below, it worked fine for me. It is imperative that you make absolutely sure that 12volt connections to the speedo are connected the correct way round or damage may result to the speedo. You would need a 12volt supply, perhaps a drill battery or similar. One other point whilst running the test the speedo is clocking up miles or kilometres on the odometer that your car has not done so you would not want to run the test for any longer than needed for a quick check.

Here is a link to a document that provides pin number details for the speedo connections.

https://www.jbugs.com/store/images/...Jqt18Ri3cKR17e4rlaA8GT8l9VtiHVsqWl4_qzN8


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Regards Tony