MapMyRide is notoriously inaccurate for cumulative elevation gain. Same goes for all the other free mapping web sites. I trust Garmin (units with barometric altimeter) a lot more, if you're sure you let the unit calibrate before starting your ride.
And no, MapMyRide does not consistently underestimate elevation gain. Their data is just not detailed enough (especially in areas with lots of steep hills) and their numbers can be very high or very low. For example, one of my favorite bike routes includes a bridge and a tunnel. MapMyRide does not count either, so they include the elevation gain over the top of the mountain (even though I took the level tunnel) and also the elevation down and back up the gorge (even though I took the level bridge). Even if your route doesn't have tunnels or bridges, if you ride a road that is near a ridge or canyon, they may use elevation data from the bottom of the hill or the top of the hill, even though your actual route is neither. Their errors continuously add up since elevation gain is cumulative, though they do use some smoothing algorithms to try to reduce the error accumulation.
Last edited by johnny99; 06-29-12 at 06:05 PM.