I experienced a case this past summer where my GPS unit (Garmin GPSmap 60CSx) registered 48mph for a segment. I know that I was nowhere near 48mph on that segment (or any segment for that matter). I was in downtown Pittsburgh at the time and think that the buildings caused the unit to become temporarily confused (switching satellites combinted with ± 20' error). I agree with the poster that said that a "more" traditional cyclocomputer (properly calibrated) would give a more accurate measure of speed.