Hello there @marco_c, it's nice to see you around. Thanks for the details provided.
Fitbit devices calculate distance by multiplying your walking steps by your walking stride length and multiplying your running steps by your running stride length. We estimate your stride lengths using your height and sex. Track one or more runs using on-board or connected GPS, we evaluate that data to automatically update your running stride length. For best results, run at a comfortable pace for at least 10 minutes.
If you prefer to measure your walking and running stride lengths yourself and enter them manually, see How do I measure and adjust my stride length?
Note that when you're tracking an activity with GPS, Fitbit calculates your distance using GPS data rather than steps. If you begin moving before you get a GPS signal, the tracker will calculate distance using steps and stride length as described above until a GPS signal is found.
Note that your device uses your step count to calculate distance until it finds a GPS signal. In this scenario, your total distance calculated for the workout may be slightly less accurate than if GPS data were available for the entire duration. For more information, see How accurate is GPS?
After reviewing this factors, please go for another run and see if the distance improves.
Keep me posted, I'll be around.
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