In my experience mapmyride severely understates elevation gain, compared to any GPS in our riding groups, MotionBased, or SportTracks correction.

The ride I did on Saturday was 4100 feet according to my Garmin 705, 4400 feet according to another rider's 305, and I believe 1700 (maybe less) according to mapmyride. The rider with the 305 was of the opinion that his GPS understates elevation gain by about 10%. Unfortunately my data file got corrupted, so I wasn't able to upload it into SportTracks, which IMO is the most accurate of all.