Whilst GPS on the face of it is not really accurate enough for position deltas at low speeds, as long as you have an app that damps and averages the results, I believe you can get a very good indication when travelling at a constant speed for a few mins (to let it get a good running average). I feel comfortable that one decimal place (in MPH) accuracy at constant speed is believable, but perhaps that it is just me that believes that.
From the BCN Challenge, my speeds indicated by GPS have tied in well against the time measurements between junctions, with the distance between junctions measured from Google Earth (I don't think the canalplan distances are that accurate sometimes).