If the safe takeoff and climb speed is around 35mph and the plane only reaches .8 to .85 of the maximum pitch speed then we're in some trouble. If we allow for an inflight max pitch speed of 50mph instead of the ground based RPM and pitch of 42 mph at .85 efficiency we've only got a top level flying speed of 42.5 mph again.
This is a common misconception. That '
85% efficiency means the airplane is only going 85% of the prop pitch speed'. It has nothing at all to do with it.
The widely quoted 80-85% prop efficiency is just a measure of how much of the engine's power is transferred to the prop, and is only useful for figuring out the change in top speed if a more or less powerful engine is used with another prop of about the same efficiency. Power relates more directly to the ability to climb as speed is related to the ratio of power to the 1/3 power.
A well matched prop generally moves the airstream of the prop blast about 10% faster than the airplane's actual speed. That roughly 10% higher speed is due to the thrust achieved which also equals the drag.