The Track log I was referring to in post #2 is the log stored in the GPS's memory
How does the 'software' read the GPS ?, if this software is constantly reading data from the GPS in real time it is probably reading NMEA sentences, these send data every second containing location, time etc. So speed could be calulated in the GPS and sent in an MNEA sentence to the 'software' you refer to in post #3, in this case the 100 MPH will also be stored in the GPSs internal memory files, however I think it is much more likely that the 'software' is reading NMEA sentences that give the GPS's location and that the speed calculations are being done by the 'sofware', in this case either the software is creating the speed error or the NMEA data is being corrupted in some way which makes the 'software' get the speed wrong