Geez I hate to post this but here goes. It’s like NASCAR drivers establishing their pit road speed by the rpms shown on the tachometer in a specific gear.
I’ve driven a car with a broken speedometer but a functioning tachometer. I determined speed by rpm readings and the gear I was in from prior experience in the car.
that I've done; now have you ever driven a stick shift with the throttle jammed wide open by switching off the engine at stop lights?
” It’s like NASCAR drivers establishing their pit road speed by the rpms...”
Sure, but the road isn’t moving as is in the air.
For a stick shift, there is a fixed relation ship between RPM and road speed, as determined by the gear you are in, caused by fixed mechanical linkage from the engine to the transmission to the wheels to the road. There is no such fixed relationship for an aircraft and airspeed.
How does using a tach work as a speedometer when the car is hydroplaning? That's closer to what happens in the air.