My friend and I both have a Garmin 205 that we wear when we run. We run together pretty much side by side, so when we compare our histories on the Garmins, we have the same pace and same distance. The only thing that's different is the calories burned, and it's VERY different. I burn more calories than she does, and that's consistently true no matter how far we run. For example, today's run was 5 miles; I burned 938 calories, and she burned 521. So my question is... why? What are the factors that determine how many calories you burn on a run?