I hope I'm posting this in the proper place... hopefully the developers of the official Nokia software read this.
I run a Web site (http://www.gpsvisualizer.com/) that draws maps from GPS data files. Many people upload .gpx files to my site that are tagged with "Nokia Sports Tracker" as the creator. However, these GPX files have two flaws in them: first of all, in the GPX version 1.1 specification, neither <speed> nor <course> are valid tags. Running a NST .gpx file through an XML validator produces an error. If you want to use <speed> or <course>, you should mark your files as GPX v1.0.
Second, if you DO want to use the <speed> tag (in a GPX file marked as v1.0), the units should be m/s, NOT km/h. It's weird, I know, but I don't make the rules, I just follow them: see http://www.topografix.com/gpx_manual.asp for the official specs. As it is, NST .gpx files get interpreted as if the speed values are off by a factor of 3.6.
There are ways to include speed info in GPX 1.1 files, but they involve creating another XML namespace with custom tags, etc. I don't know enough about XML to be able to articulate the details...