Backpacker
I meant - the decision to trust the ASI rather than the tachometer.
I don't agree with you about it being the wrong decision not to continue, either. I had to make a quick assessment of the situation. Adjusting the power to 2200 rpm in cruise meant I was unable to maintain height. So what does that tell me? Obviously if there was any other indication of the engine being outside limits it would have been a different story.
Without any other indication of engine malfunction and given that the flight was short in any case I didn't see that returning to the departure airfield immediately was necessary.
I can't believe that you really want me to fly by RPM readings on final approach? Especially considering that all the evidence pointed to the tachometer reading incorrectly. I know it's possible to fly without the ASI on final, but honestly - air speed is absolutely critical at this stage.
Being PIC means having the privilege and responsibility of making a decision and standing by it. Sometimes with hindsight and more information one discovers that a better course of action could have been taken. Not this time!