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.
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.
It urged people to research any device before purchasing and warned that buying a well-known brand did not guarantee accuracy.
Natalie Hitchins, head of home products and services at the consumer watchdog, 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.”
In response to the research, Garmin highlighted that the Vivosmart 4 does not use GPS and was therefore not their recommended device for marathon runners.
“The Vivosmart 4 is an all-round smart fitness activity tracker used to monitor wellness, health and fitness for a range of activities,” a company spokesman said.