Some fitness trackers are inaccurately measuring running distance, according to research from the consumer watchdog Which?
It tested 118 trackers using a treadmill to complete the distance of a marathon – 26.2 miles (42km).
It found that the least reliable was the Garmin Vivosmart 4, which underestimated the distance by 10.8 miles – meaning the researcher actually ran 37 miles.

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