Sorry I couldn't read all the 28 pages so please disregard this if already discussed.
There is no question that the GPS on Fitbit trackers (at least Charge 2 and 3 that I own) is way off. As you keep using the device, it relies on the distance travelled (initially captured by the GPS) to calculate/adjust the running stride length (distance in inch divided by step count) and then it increasingly uses this measure to calculate the distance and pace of your running. Since the GPS is inaccurate, the calculations will only get worse over time in a positive feedback loop:
Distance is underestimated --> Running stride length is reduced --> Distance is further underestimated --> Running stride length is further reduced --> and the same loop goes on...
I do a 3 mile loop in my neighborhood every other day and I've seen it in action as it started from reporting around 2.5 miles (incorrect to begin with) and gradually went down to around 2.2 miles for the same loop.
Here is the work-around (not a real solution until Fitbit addresses the faulty GPS in these devices): Go to Advanced Settings, then Stride Length and disable "Set automatically" and enter the value manually. To find your stride length, run known distances for a couple of time and divide the actual distance (in inch) by the step count from the Fitbit and average it out (or use metric units if applicable). I did it and found my average stride length to be 44.2 inch (the automatically set value had gone down to 39, an over 12% underestimation). I then did an official 10K (6 mile) run competition and the Fitbit calculated the distance almost accurately to be 6.01 mile.
I created a spreadsheet and put my step counts and known distances there from time to time to make sure my manually calculated average stride length remains accurate and so far haven't seen the need to change it from 44.2. Sad that I need to be doing this myself when I paid for a device that is supposed to be able to do this very basic task itself, but it is working for me for now...
