I've found the Map my Ride and Strava apps to be flakey with both distance and speed. They tend to take individual position readings as gospel, rather than doing any kind of averaging over a few readings. I've had a 5km distance discrepancy over 25km when walking under trees as the position jumped around, it added all the jumps together as distance walked. They both need a re-write of their code to do more averaging. I have another app that is just a GPS speedometer, it has an option to determine the time the speed is averaged over.