When I run I've been using the outdoor run feature on the watch along with starting ismooth run on my phone. The pace shown on the watch and the app are usually noticeably different. Yesterday I brought my Garmin Forerunner 15 along as well. The Garmin and the app differed by 2 seconds for a 3 mile run. The Apple watch was off my almost a minute. Periodically during my run, I'd look at each and they were never the same and often very far apart. At this point, do I need to unpair and recalibrate or can this be done without unpairing? I hate to unpair as fear I then have to set up Apple pay again. Oh, the distance is off too by about .10 of a mile.
Watch will drift in accuracy. Some runs it will be dead on but other times if can be off by a fair percentage. Yesterday I ran 7 miles and it was 7.03 on my Garmin 620 and 7.00 on my