Analysis by consumer watchdog Which? said fitness trackers from brands including Garmin, Fitbit, Apple and Huawei were unable to accurately track someone running a marathon distance.
The research has been published ahead of the London Marathon, which takes place tomorrow.
Garmin’s Vivosmart 4 was named the least reliable by the Which? research, which found that by the time the device had clocked the marathon distance of 26.2 miles, runners would have in fact run 37 miles.
Samsung’s Gear S2 was also found to miscalculate the distance according to the research, saying the wearer had reached marathon distance only after they had run 36.2 miles.
The Misfit Ray, Xiaomi Amazfit Bip, Fitbit Zip and Polar A370 were all also named as devices which carried runners past the 30-mile mark in testing.
Huawei’s Watch 2 Sport left runners short of a full marathon, the research claimed, telling the wearer they had reached the milestone after only 18.9 miles. And the Apple Watch Series 3 said the target had been hit at 22.8 miles.
Which? said it carried out its analysis using a calibrated treadmill to compare the ability of different trackers to log steps taken and distance travelled.
Natalie Hitchins, head of home products and services at the consumer watchdog, said: “Running a marathon is no mean feat, so runners will want to know their fitness tracker is not jeopardising their times.”