I am wondering if theres a fix for this- 4 people leave at the same time from the same place and do the exact same ride at more or less the same speed, start and finish together. Garmin 820 shows feet climbed at more than 200 ft difference between two of the riders and none of them are the same. Mileage is the same within a tenth of a mile so its not a gps problem. I believe the garmin uses atmospheric pressure to measure climbing. Anythoughts?