I'm having issues with max speeds uploaded from my Forerunner 405, both in GC and after exporting.
An example activitiy: http://connect.garmin.com/activity/513186084
Max speed on the GC activity page is listed in the sidebar as 38.1 but doesn't show anywhere about 34.5 on the timing chart. Worse, the exported tcx file is read into other applicatios with a max speed of 137.1. While some of the blame might be with the other applications, the max speed is reported incorrectly in GC and elsewhere so I think the way GC is reading/writing the files is faulty...