True, i know about rpm, use it a lot on landing, when rtb and low on gas
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What I mean is with your engine shot, damaged, out of oil and so on and your prop is clearly NOT spinning ( it often stops with one blade in front you cockpit ), the dial still reads the last rpm you were flying with.
I think you can test it when making a bad landing when your prop hits the tarmac but for the rest your plane is still intact. The dial will read the last rpm you flew with.