"RG" wrote ...

Speed is calculated by using simple time/distance calculations, measuring
the time it takes to get from one known fix to another. It is common for a
GPS to get an errant fix from time to time. Sometimes, these errant fixes
can be quite a ways off, but are usually only momentary. Nevertheless, they
are included in speed calculations. The seemingly unbelievable max speed
results from the very fast side trip your GPS thought you made while
visiting the errant fix. I've seen max speeds in the many hundreds of MPH.

That makes complete sense but for what it's worth my Garmin GPSMAP 168 (external
antenna) has never done this. The max speed recorded is 43.something mph and
I've watched the speed go over 42 steady state. Different software perhaps.

-rick-