I need to swap Garmins with you, I guess.

Last week, I did an 80 miler on Wednesday, 60 miles of it with another rider. My ending elevation, 3,250ft. His, 3,350ft. So apparently, I managed no elevation change in those 20 solo miles. Actually, negative 100ft.

Looking over the numbers, I seems I get a consistent average of elevation under-reported ~5%, and distance by 1%. Sounds like nothing, until you extrapolate it out over the past couple years-- Strava has eaten about 175 miles and 38,000ft of climbing. Jerks.