Sorry High Planes the story was in another thread ( Could sound barrier be broken with a prop plane ? ).

http://www.eaa.org/experimenter/arti...02_elippse.asp

Insert: Testing of the fixed-pitch, three-blade ELIPPSE prop on my Lancair shows that it is performing at 82 percent efficiency in a climb at 105 mph IAS and at least 90 percent efficiency in cruise at 200 mph TAS.

90 % it says at TAS 200 mph.