The Wright brothers did not just think, based on calculations, that their propeller was 66% efficient, they claimed they had built a 66% efficient propeller
"Our first propellers, built entirely from calculations, gave in useful work 66 per cent. of the power expended. This was about one third more than had been secured by Maxim or Langley."
Source: "The Wright Brothers' Aeroplane, O. and W. Wright, The Century Magazine, September 1908, pag. 648-649,
The Wright Brothers' Aeroplane [Orville and Wilbur Wright, The Century Magazine, September 1908] | Library of Congress"