Since renting a tool is a hassle, there are also these very affordable non-contact tachometers:
Amazon.com: CyberTech Digital Photo Tachometer Non Contact Tach: Industrial & Scientific
I have used one for two years to adjust idle and they work very well. I just wish there was a way to mount them and view the display remotely. I would love to know rpm vs mph for my little 220d.

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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).