Maybe a noob response, so be patient with me. On the speedo question, in all the cars/trucks I've worked on in the past, that was controlled by the speedo gear in the trans. I have zippo experience with the process in a Mercedes car, but we always checked the odometer accuracy by attaching to the speedo cable where it enters the instrument cluster. It didn't matter what make or model we were working with, foreign or domestic, everything turned the speedo cable x rotations per mile. FWIW, my experiences included medium-duty Mercedes trucks.

If MB does it differently, then it has to be in the speedo/odo unit where the speedo cable attaches to the instrument cluster. Assuming that is the case, couldn't you simply pop off the speedo needle, swap the speedo/odo cluster from the 2.88 car (resetting the odometer numbers to match the current 3.08 numbers), assemble the 2.88 unit into the primo instrument face, reattach the primo needle, and be done?

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Ken

1990 350SDL (It's a work in progress) 250K mi
1991 350SDL that was SUPPOSED to be a parts car 323K mi
2000 F350 Powerstroke 180K mi, 100K mi on B20 - B99
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Gone but not forgotten
1982 300D
1991 Jetta ECO Diesel

This has been known by many for years. The gears in the transmission are the same across all years/models. The speedometer/odometer is based on the differential ratio.

That, or painting all the needles the same like what was suggested before.

Add another member to the rear headrest club! Mine came with them as well.

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1984 300D Turbo, 326000 Km, Thistle Green Metallic