ORIGINAL: buzzingb
A 6" prop has to spin at 17,600 to reach 100 mph without any loss or drag, so for speed you might look at a 8" pitch.
Dont be so sure.... There is more to a prop than its pitch - its not 100% related to the flat surface pitch
A Q-500, TT40, 9x6 prop, 16,500 at launch ... lets say maybe 17000 in flight --- does fly at 120mph
By your calculation there this is impossible. But it is known to be true.