Does anyone have a feeling about how accurately Map My Ride calculates hill grade percentages? I don't have GPS so I can't cross check. I just mapped some of the hills on my local route and I'm kind of surprised by what it came up with.

My most hated hill came out to 20% grade () at the bottom, flattening out to between 15% and 17% as the climb continues. My heart rate really kicks up on that hill, so I assumed it was a pretty good percentage grade. Another steep section came out to 19% and several other hills calculated at between 12% and 15%. If these are accurate, then I know why I need my granny gear!