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.
Garmin said it was because that particular tracker did not contain GPS.
It described the Vivosmart 4 as an “all-round smart fitness tracker” and suggested that marathon runners use its Forerunner range which is GPS-enabled.
Of the eight Apple models involved in the test, the Apple Watch series 1 was the most accurate, over-estimating the distance by 1%, while the series 3 overestimated by 13% – stating that the runner had completed the marathon distance after 22.8 miles… More here.
I have tried and used trackers from Apple, Fitbit, Garmin and others over a few years now and not one of them has proved to be anywhere near accurate.
There are obvious reasons for this because stride length cannot be measured accurately and it will change depending on what you are doing. However, saying that the lack of GPS causes a problem is not a good thing because the tracker in question should not be offering to track treadmill running and other activities in the first place.
Trends are all you can look at. Accept the inherent inaccuracies and look to build the trend over time.