Prop efficiency?
tricky one, that.
Why?
In actual practice, it is firmly related to the airframe being dragged along.
or worse - pushed along (ever tried to push a piece of chain?)

Example:
a prop which reaches a theoritical 85% of it's possible screwing speed- may do so in a given super slippery airframe.
Now put same engine and prop in a big draggy airframe -
It will never reach that 85%.
So - prop efficiency should always be a statement of prop to airframe relationship . (efficiency)
No/Yes
In my opinion most prop charts are best used -if printed on soft , double ply tissue.