These photo tachometers are cheap and if you think your car's tach is off, would give you a way to tell for sure. It would be hard to use one while driving, but parked with the hood open, the work really well. You stick a small reflective piece of tape (included) to the crank and point the tach at the crank. Maybe a friend could sit in the car and hold the rpm at some elevated level, and you could verify the actual rpm compared to the indicated rpm.
CyberTech Digital Photo Tachometer Non Contact Tach bundle CyberTech Cable Tie: Amazon.com: Automotive

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).