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Ahead of this weekend’s London Marathon, Which? has just released data which shows that inaccurate fitness trackers could leave runners miles from the race finish line.

The data reveals the dramatic difference between actual distances run and those reported by popular fitness trackers and wearable tech. Which? tests have found that some trackers can be wildly inaccurate – over or underestimating distance covered by up to 32%, which on a marathon route could mean you miss the finish line by eight miles.

If marathon runners relied solely on wearables for accuracy on distance covered, many would not cross the finish line, and some would actually end up running an even greater distance! The data shows that the most and least accurate brands are:

Fitness brands

Marathon distance at 26.2 miles

Percentage difference (whether above or below 26.2 miles)

Samsung

22.55

-14%

Polar

25.17

-4%

TomTom

25.17

-4%

Fitbit

25.43

-3%

Misfit

25.69

-2%

Garmin

25.69

-2%

Huawei

25.96

-1%

Withings

27.53

+5%

Apple

28.05

+7%

Pebble

30.68

+17%

Note: Data is based on the average for models tested by the brands listed, based on the accuracy of distance calculated based on steps.

The full study data is available on the website where you’ll find a list of the most and least accurate devices (by distance based on steps).