More simple but not as technically correct-- but probably as accurate because of real life factors(true pitch of prop ect) is to take the pitch X RPM. Here is how close you get to the above formula. 10000rpm x 4pitch=40mph....close to 37.88

Calculating your 9300 x 6 = 55.8 mph--this is the theoretical speed of the column of air pushed by the prop at static if that is when you took your RPM reading.

The motor might unload to 10,200 in the air 10,200 x 6 = 61.2mph then subtract drag of air craft???my guess 30%(I have no basis for this it is only for example 30%)--(this drag will increase expenantaily with speed) or .3 x 61.2 =18.36 61.2-18.36 = 42.84mph.

Give us the Radar gun results when you get them.