Garmin 910 vs MapMyRide

The algorithm that mapmyride uses to calculate elevation is terrible. Go with your Garmin. It's not perfect, but it a lot closer to reality.

iPhones & smartphones are not dedicated GPS devices.  As such, they are only as good as the app & phone antennas.  I would put more faith & trust in your Garmin over any app.

I used mapmyride to determine my elevation training for IMCDA in 2011. I had a damn good bike split because I had done more than 2X the elevation training than I thought I had. At least there are some benefits to how off it is.

I have learned not to trust MMR, although I have no experience with Garmin. One time I mapped a route that showed a 3-4% hill, and it ended up being so steep that my front tire was lifting off the ground and I had to zig-zag to get up it.

+ whatever we're at to MapMyRide being awful. It's useful to compare itself to itself (like this course it hillier than that course) but useless otherwise. Strava and 910 seem to be close to each other, so I guess they're better. Since I've got no reference point for any of them, it's hard to say for sure. THe Garmin has also had it's moments of wild inaccuracy.

John

I open it up later in garmin connect and turn on elevation correction, then get start and finish elevations and distance and punch those numbers into a grade calculator. Average grade will always be more accurate than instantaneous (unless it is super sporadic).

If you're really concerned about elevation accuracy during your ride or post ride, get a garmin (or computer) with a barometeric altimeter.  They work extremely well.