These are interesting comments.
Ghengis:
(1) TAS v. CAS: Yes, but hardly so as to have such a large difference as I observed?
(2) GPS lag: I did "creep up" slowly on the stall each time. Definitely the stalls were approached at less than 1 kt/sec decrease. GPS speed was observed constantly (by the passenger) and stabilised. I am convinced GPS lag cannot have been a factor, and even if I were wrong, its effect could perhaps be 1 or 2 mph, not 15 or 20.
(3) and (4) Yes the ASI is in an position at stalling AoA which is likely to give inaccurate readings. If one starts from the premise that the GPS does not lie in respect of speed over the ground, then the inaccuracy must be in the ASI. This particular GPS has been tested for accuracy of its speed readings, on the ground.
(5) I did say I was near max weight, probably about 98% of MTOW. By your own formula this would make an unrecognisable difference to the stalling speed.
(6) I covered the point about the vertical speed component in my previous post. For a rate of descent of say 500 fpm or even 1000 fpm this cannot account for such a large difference, as a simple calculation will show.
(7) No, there is no confusion of units here. The ASI reads mph as also does the GPS, and the book figures are given in mph also.
And your further comments:
Wind condition: This test was deliberately done on a day with almost calm conditions. There was a small difference (2 mph or less) in the up/downwind stalling ground speed.
Power: Power was slowly brought back to idle as the airspeed decayed.
CofG: I mentioned that this was near the mid position.
Use of non-aeronautical units: ???