Oh sorry. I meant to add a few comments about your question about Doppler speed measurement.
When we first started way back in about 2004, the only GPS data we had acces to was 'trackpoint' data. In other words, data about where we were in space at a particular moment. Our precise location every second. Unfortunately, we soon found that the data was all but preciseand we were often were not where the data said we were. There are many inaccuracies built into the location data that can make speed measurement very inaccurate. Thankfully, a couple of clever guys, Keen Rogers and Tom Chalko realised that some GPS were capable of recording the speed over ground calculation from the GPS system using Doppler shift. They have found this method of measurement to be massively more accurate than using the trackpoint locations. This is why we use the Doppler speed measurement in preference.
Regarding your results: The very short time measurements like 2 seconds are much more subject to trackpoint error than longer periods where the errors tend to cancel themseves out to some degree, but not entirely. Sorry to tell you, for top speeds, your Doppler speed is far more accurate. :-)