Adjustment for the mechanical VDO Speedometers.

So for the last couple of days I’ve been trying to keep my original speedometer to use with my new 2.88 ratio differential on my w123 300D. The speedometer is calibrated for a 3.07 ratio differential. I do have a 2.88 speedo, but I am opting to keep my original because it is in such good shape.

Although I am not an expert about this and correct me if I am wrong, but what I know about these mechanical speedometers is that it runs off a circular magnet that spins in a speed cup. As the car’s speed increases, the magnet spins faster and creates a magnetic field that act to spin the speed cup. The speed cup is connected to a hair spring and then to the needle. The hair spring provides enough resistance to the keep the speed cup from completely spinning and keeps it at certain marked speeds. The calibration of the speedometer can be made by either adjusting the hair spring or adjusting the power of the magnet. To get a more precise calibration, adjusting both the magnet and hair spring might be required. Since I have no way of adjusting the magnet (which requires a powerful electro magnet to do so), I am focusing on just the hair spring.

I basically adjusted the hair spring until my speedo closely matches the GPS. I had to do take apart my cluster a few times to accomplish this. I started with about a 15% error across the speed range, than I got a 10% error across the speed range. I ended up getting my speed accurate between 40 mph to 65 mph. Below 40 mph, my speedo error is showing at the most 4% fast and over 65mph at the most 4% slow. Not sure why it wasn’t consistent across the speed range like my other two % errors.

But that is good enough for government work and then that is good enough for me.

This spring adjustment might be particularly useful for someone doing the 2.47 diff swap. I have a thought of using a weaker hairspring from an 85 mph speedo on a 120mph speedo. To make it work, you’ll probably have to adjust the tension by shortening the hair spring to make it have more resistance because the 85 speedo’s needle has less numbers to travel through then the 120 mph speedo. But that is just my thought and will need to be tested out.

Anywhoo, here are some pics of what I did.

I took the speedo out of the cluster.


Removing the needle. To not scratch or dent the speedo face, I placed a thick stack of note paper with slice in it on the face and under the needle. I then used a wire cutter to pop the needle off. It will be tight and might go flying off somewhere.. So be careful.


Hair spring. Because I was showing a speed that was slower then I was actually traveling, I had to loosen up the hairspring by lengthening it. I did so by removing the lock down pin and draw the spring down through the hole. If you run out of adjustment, then you need a weaker spring.


Here is a shot of the original hairspring and another hairspring from the 2.88. They were physically the same size. So I knew it just a matter of adjusting the tension to make it work.


As far as correcting the odometer. I swapped in the 2.88 speedo's gears. Works perfectly.




My adjustment is basically guessing. Not entirely accurate. But I hope this gets us thinking of how to correct a speedo with someone who does not have an option to simply swap out their original unit for another factory unit... like the 2.47 differential guys.

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