Before I say any more, I should mention that the information I found pertains to the early W201 series with mechanical speedometers (i.e. with a speedometer cable running up to the instrument cluster), and things may be different for the 123 series, or for newer cars (late-80s and newer) with electronic speedometers.

The website with the rear axle ratios (and much more info) was: http://www.baureihe201.de/W201/ It's a W201 site, so it may not help out people with W123s...

I e-mailed a friendly German ebay seller about a W201 speedometer he was selling. He said he bought it for his 190D, but when he put it in the car, it didn't read the correct speed. If he went 125 km/h, the speedometer only said 90km/h. His original was a 201-542-85-06 (180km/h max), and he tried out a 201-542-14-06 (also 180km/h max). He went to the dealer and bought a new one (same part as his original) and that one worked fine.

So, he tried doing a swap to another speedometer and it didn't work. And it wasn't even a different-max-speed speedometer - it had the same maximum speed and it still didn't work!

He recommended that I not buy the part from him because he thought I'd have a similar problem (it's nice to find someone honest!).

So, maybe things are different for the 123s, and things are probably also different for newer cars with electronic speedometers (does the 1989 300ce mentioned earlier have an electronic speedometer?)

I found 13 different versions of the W201 metric speedometer on eBay, and in the end I had no idea which one to get. I gave up on making this swap because I couldn't find a reliable source of information, and I didn't want to do it unless I knew it would work (shipping from Germany is expensive!).

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1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD