I don't.

That said, it can't be too wildly off; it shows my idle speed to be about where it should and responds as expected to revving the engine... My car is in an undrivable state at the moment so I can't test more than that right now.

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)