Lance, the drop in stall speed was measured by GPS. We had used the GPS in the past under almost the same conditions (two on board, full fuel, same weather) BEFORE we installed the VG's. Then a few days after the VG's were installed we did 'landing practices' again. This time, the airspeed was at least 5 mph slower on the GPS than the GPS speeds before the VG's were installed. (We did this because Pete wanted a benchmark speed with which to test the claims of the VG manufacturer, and we spent two houroed, and besides, I use the a/s indicator like I use the speedo on my land based vehicles: they are a guesstimate 9within a certain percentage) as to my true speed. My Honda motorcycles have a 10% speedo error above 25 mph. The speedo will read 10% higher than what the GPS is telling.