With the London Marathon 2019 around the corner, we've laced up our trainers and put more than 100 activity trackers and smartwatches to the test. The least accurate based on distance while running underestimated by more than 40% - in a marathon (26.2 miles) that equates to almost 11 miles.
After crunching the numbers in the Which? test lab, we've highlighted some of the worst wearables across popular brands for under or overestimating distance while running. If you're using one of these while training for a marathon, you could well struggle to pace yourself on the big day.
Our graphic shows that there was a significant difference in accuracy between models. For each of the eight activity tracker brands we looked at, we've highlighted the worst product for underestimating or overestimating distance while running.
Apple did relatively well in our tests, but the Apple Watch Series 3 (GPS) was its worst performing wearable for accuracy here, overestimating distance covered by 13%. If the Series 3 says you've run 26.2 miles, you've actually run 22.8 miles. Perhaps surprisingly, the Apple Watch Series 1 only overestimated distance while running by 1%.
Other models didn't fare so well: Samsung's popular Gear S2 smart watch was 38% out, and even household fitness name Fitbit missed the mark by nearly five miles.
Brace yourself, because the Garmin Vivosmart 4 will have you running further than you need to. When the £100 Vivosmart 4 thinks you've travelled 26.2 miles, you've actually gone 11 grueling miles beyond that - around 37. You deserve a rest.
Garmin told us that it recommends choosing a tracking device from its dedicated runner Forerunner range, whichhas in-built GPS, rather than theVivosmart 4, which doesn't, and which is aimed at those wanting to track a range of activities. If running is your main form of exercise, a Forerunner could come in handy - check out our to see how the Forerunners perform.
If you're using the Huawei Watch 2 Sport to track your long-distance runs, you'll find the ground covered has been overestimated. If this fitness watch reported 26.2 miles travelled on marathon day, you'd actually find yourself around the 19 mile mark near Canary Wharf.
Huawei told us that results may vary depending on test conditions and runner variances, that it is committed to providing an accurate and better running experience for users, and will continue to work to optimise existing and new generation products.
Distance accuracy while running is just one of a range of metrics we test to determine the very best and most accurate wearables. Distance accuracy while walking can actually vary dramatically from this, and we also test step count accuracy, and calories.
One of the most basic uses for smartwatches and fitness trackers is counting steps - it's a helpful way to get an overview of your activity and set some simple exercise targets. Hitting that magic 10,000 isn't easy though, and it's even tougher if your wearable isn't giving you accurate data.
If losing weight is your main goal, you're going to be interested in how many calories all that hard work is actually burning off. Be careful if you're relying on a wearable though - we've seen some pretty shocking inaccuracy here as well:
We go the extra mile when it comes to testing fitness watches and activity trackers. Our expert tests sort the diamonds from the duds, with a strong focus on accuracy and other important features that matter to people the most: