I have created a GPX file from a non Garmin device that I like to include into my Garmin Connect activities. However, the speeds that are calculated from the file are way too high. I was doing a test drive with a car. I know that I did not drive over 100km/h but in the Garmin Connect speed graph I see a point at which the speed is 142km/h!
It looks like speed calculations are way off!