Fitness slackers? New survey reveals worst distance trackers ahead of London Marathon


According to Which? the Garmin Vivosmart 4 underestimates the distance you have run by over 40%

New analysis from Which? has exposed the least reliable fitness trackers for measuring running distance – with one tracker underestimating the distance of the London Marathon by an astonishing 11 miles!

Ahead of this weekend’s London Marathon, Which? tests have uncovered the most inaccurate fitness trackers from big brands including Apple, Fitbit and Garmin – with the least reliable tracker underestimating distance covered by as much as 41.5%.

The worst offender was the Garmin Vivosmart 4, which underestimated the distance of a full marathon by 10.8 miles. Which?’s tests found that when the Vivosmart 4 tells you that you’ve run 26.2 miles, you’ve actually gone almost 11 miles beyond that – meaning those training for a marathon would be actually be running around 37 miles in total!

On the other had, a runner using the Huawei Watch 2 Sport could find themselves around seven miles short of the finish line of the marathon, with Which? finding this model overestimating the distance covered by 28 per cent.

Relying on the Watch 2 Sport would leave a runner finishing around the 19 mile mark, or near Canary Wharf in this weekend’s marathon – one of the largest overestimations of distance of any wearable in the study.

Apple fared relatively well in the Which? tests, with the smallest difference between its most and least accurate models. Of the eight Apple models we tested, the most inaccurate was the Apple Watch Series 3 GPS, which overestimated distance covered by 13 per cent, or around 3 miles under the finish line of a marathon. The best Apple model – the Apple Watch Series 1 – only overestimated distance while running by 1 per cent.

Which? testing has also shown considerable variations for tracking running distance within brands too, suggesting that buying a fitness tracker from a well-known or reliable brand doesn’t guarantee accuracy.

For example, Garmin has produced several good models – the Vivoactive 3, for example, was 100 per cent accurate at tracking running distance in our tests. Yet the Garmin Vivosmart 4 underestimated distance covered by 41.5 per cent, and the Garmin Vivosmart HR overestimated by 30 per cent.

See below for the least reliable tracker, based on distance while running, from each of the eight biggest brands:




% Inaccuracy

Actual distance covered by marathon runners


Vivosmart 4


37 miles


Samsung Gear S2


36.2 miles


Misfit Ray


34.6 miles


Xiaomi Amazfit Bip


34 miles


Fitbit Zip


30.9 miles


Polar A370


30.9 miles


Apple Watch Series 3 (GPS)


22.8 miles


Huawei Watch 2 Sport


18.9 miles


Natalie Hitchins, Which? Head of Home Products and Services, said:

“Running a marathon is no mean feat, so runners who are putting in the months of training beforehand will want to know their fitness tracker is trustworthy, and not jeopardising their finish times.

“Our tests have found a number of models from big-name brands that can’t be trusted when it comes to measuring distance, so before you buy, make sure you do your research to find a model that you can rely on.”

A spokesperson from Garmin said:

“The Vivosmart 4 is an all-round smart fitness activity tracker used to monitor wellness, health and fitness for a range of activities. As the Vivosmart 4 does not incorporate GPS, our recommendation for someone who is running long distances such as a marathon, would be to choose a tracking device such as our Forerunner range which is dedicated to running and incorporates GPS”

Chris Price