If connected GPS is not used, your watch measures speed and distance using the built-in accelerometer, a motion sensor that detects movement from your wrist. To make measurements as precise as possible, the accelerometer needs to be calibrated, because wrist movement and stride length varies from person to person.
Learn in the following FAQ article how to calibrate your Suunto 3:
Why do I need to calibrate speed and distance measurement in my Suunto 3?
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Suunto 3