Some may have seen the discussion on GPSTC recently where Boro got substantially higher speeds on the GW-60 than on his GT-31s. The conclusion was that this was due to poor GPS reception when he had the watch on the front hand and used an under grip during speed runs. I have been able to reproduce this during a simple test drive with 6 GPS units (GW-60, GW-52, and GT-31).
It has been suggested here many times not to use the watch together with an under grip on speed runs, or to wear the watch to the inside, or (even better) to wear it on the arm or on the helmet. What surprised me somewhat, though, was to see that the error was not random - at times, the speed would deviate in the same direction for 10 seconds or longer. I posted details about the issue at boardsurfr.blogspot.com/2017/05/the-fastest-way-get-faster.html