400bhp
Guru
That's right - the phone isn't going to be as accurate as the Gramin with the speed sensor.
Also it's easy for signal to be lost, and we know it's not exactly acurate on position, so that few feet difference would equate to those couple of seconds.
Well, I'm not sure on that - it depends how Strava calculates GPS position and time travelled between 2 points. My guess it takes the easy way to do it and just uses the GPS positions and time, rather than trying to work backwards from the sensor and distance travelled, then convert that to a GPS position.