I've had my Series 0 Apple Watch Sport since April 2015 and, aside from the initial calibration when I first got it/each time I've unpaired and repaired the watch to my iPhone for troubleshooting, I normally leave my iPhone at home while I go for a run/walk the dog. However, I've been taking my iPhone with me the last couple of months on my dog walks, and the amount of calories my Apple Watch records is way off from it's been telling me before. I have a fairly consistent route (and it measures distance accurately whether I have my iPhone with me or I'm just wearing my Apple Watch), and the Active Calories metric is impressively accurate compared to workout machinery the few times I end up going to the gym (elliptical and treadmill are consistent with my Apple Watch's measurement to about 10 calories over 30-minute workout). It's just that when I go for an outdoor workout, the difference between the Active Calories record with the Apple Watch and iPhone is 150+ calories lower than when I workout with just the Apple Watch (e.g., about 170 calories vs. ~300 calories over the same 50-minute walk). I've tried recalibrating it, as well as unpairing and repairing the Apple Watch to my iPhone, several times, all to no avail. I've been unable to find any articles online about this, but I'm wondering if anybody else has experienced a similar issue?