It occurs to me that I should add that I had a similar experience with one of my tachometers on my '73 P. Both were tested by my A&P with a instrument that he trusted (and I trusted him). The front tach was right on, the rear was 50 RPM high. Instead of recalibrating the tach, we just placarded it, and I flew it indicating 50PM lower than the desired RPM.
Kevin
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