Backpacker
I would expect to see about 2200 rpm in the cruise. That's when I first noticed the tach was out (should have noticed it earlier, but as I have said, I had no previous experience or instruction on this kind of fault - it was totally unexpected). Of course, as you say it could have been the ASI that was overreading, but given the sound of the engine, performance and so on I decided the tach was the faulty instrument.
Mistakenly assuming the tach as correct (which indeed is what we are taught) and suspecting the ASI could in this case have lead to a stall on final.
I made the right decision.
Any instructors out there - maybe make a note to mention the red-line issue to your students?