W123 Speedo reading way low. Aftermarket retrofit options?

[efm-7]

Good afternoon folks!

When I bought my 4-speed manual 240D, the speedometer didn't work at all.

Crawl under the car to see that the speedo cable was not plugged in, (I shudder to imagine now many undocumented miles there are on this thing lol).

I go to plug it in, but the cable has a flat pin whereas the tranny wants a square pin. As a temporary measure, I ground down the flat pin until it fit into the tranny.

With the car up on jack stands, I put it into first gear, give it a bit of gas, and while I see the odometer reading going up steadily, the speedometer does NOTHING until I have my foot down in 2nd gear.

Research on the internet made me conclude that perhaps the magnets in the speedometer may have weakened and thus I need a new speedometer. Problem is, such a thing is very hard to find here is very expensive for one that might or might not be in worse shape.

However, my other problem is that I don't know for sure that the speedometer is the problem. From the looks of it, this car used to be an automatic that was converted to manual so maybe the speedos are calibrated differently on the auto models? Again, not even sure the cluster is original to the car either. My diff gearing is 3.69.

Rather than trying to make the current speedometer work, I wonder if I can install an electronic sender somewhere and use an aftermarket gauge that can be adjusted on the fly.

Any insight on this? I don't know which course of action is more realistic. I am convinced that my current speedometer is just worn out magnetically. But before I go looking for a replacement on evil bay, I'd like to investigate other potential causes to the issue as well as other solutions.

I would be happy with using my iPhone as a gps speedometer, but that's not likely to fly for an MTO safety inspection.


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