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Marathon training: should you trust your fitness tracker?

We've found inaccurate wearables that could make you run an extra 10 miles. Follow our tips to help you choose the best fitness tracker or fitness watch.

The London Marathon is this weekend, when around 50,000 entrants will be hitting the streets of the capital after months of training. If you’re inspired enough to start training for next year, and want a fitness tracker to help you, buyer beware.

We’ve found fitness trackers, GPS running watches and smartwatches that greatly under or overstate distance – so you may not have been hitting that 26-mile mark after all.

Blisters, shin splints, rubbing (in…um…uncomfortable places) or even a bulky pantomime-horse costume – these are expected discomforts when running the London Marathon. But what if your own inaccurate wearable device is also hindering your performance?

Here we reveal the dramatic differences between actual distances run, and those reported by the most inaccurate devices from our tough wearables tests.

Fitness tracker reviews – read our verdict on the latest devices from Fitbit, Garmin, Samsung and more.

How do wearables track distance?

There are two ways wearables calculate the distance you’ve travelled.

  • Steps – the most common method is by using the number of steps you’ve taken to calculate the distance you have travelled. Some wearables will ask for data on the length of your stride in order to improve accuracy, while others will use a default figure based on an estimate. If you can add your stride-length data, then it’s a good idea to do so to ensure you’re getting the most accurate distance figures.
  • GPS – you can buy fitness trackers, GPS running watches and smartwatches that have a built-in GPS sensor, or that are able to tap in to the GPS sensor in your smartphone.

Distance tracking – what we’ve found

We put wearables through their paces for accuracy of step tracking, calories burnt, heart-rate monitoring and distance travelled. If you’ve been training for long-distance running, then it’s likely that distance tracking is the most important feature for you.

But we’ve found a fitness-focused smartwatch that understated distance travelled based on step count by a whopping 38%, and a fitness tracker that overstated it by 30%. This means you could have been running anywhere between 18 and 36 miles when your wearable reports you’ve gone 26.

The situation is better if you’ve been using a wearable with built-in GPS, such as the Apple Watch Series 2, Garmin Forerunner 735XT or Garmin Vivosmart HR+. Built-in GPS tends to be more accurate at tracking distances than calculations based on steps. The average difference between actual distance travelled and the reported figure was just 5%, and some wearables had no error at all.

To find out which devices struggled with distance, and how they performed on other fitness tracking metrics, click through to our wearable reviews.

The Which? wearables test

Our test participants walk on a calibrated treadmill at 4.8km/h for 10 minutes, and run at 9-10km/h for 10 minutes. This means we can compare each device’s ability to log steps taken and distance travelled. For those wearables with built-in GPS, we put them through an outside run that includes sections under tree cover to check for GPS drop-outs.

Our users spend weeks with each tracker doing a variety of activities, from carrying shopping to playing sports or going for a jog. They rate each one for comfort and obtrusiveness. We look at the quality of the companion app, and the motivational tools on offer. When you hit the wall, whether that’s mile 6 or 16, the cheering crowds will go some way to keep you putting one foot in front of the other. But the best wearables from our tests could also help to keep you going.

All of this means we can recommend the very best wearable for you, whether you want to track steps day to day, or want a device for more dedicated fitness tracking.

Find out more about the lengths we go to on our how we test fitness trackers page.

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