The built-in accelerometer estimates speed and distance based on your wrist movements, so your running form, especially the way you swing your arms, is a key factor in the calculation.
Generally, this feature works best when running at a pace that feels natural and comfortable for you. When running at a slow pace your step may become bouncy, and the accelerometer can interpret your speed to be faster than it actually is. At high speeds, your running form might not be as natural, and your arm movements may become irregular, resulting in inaccurate speed and distance readings.
As a rule, any activity that interferes with your natural arm swing (e.g. pushing a stroller or walking a dog) will have an adverse effect on the accuracy.
How you wear the device
The device should be worn snugly on your wrist to avoid any shaking. To get consistent readings it should always be worn on the same position on your wrist. Avoid wearing any other devices like watches, activity trackers or phone armbands on the same arm. In addition, do not hold anything in the same hand like a map or a phone.
Make sure you’ve set which hand you wear the device on and your height correctly in the device settings.
When running on a hard and elastic surface, the acceleration values measured from your wrist are different than ones measured when running on a soft or slippery surface. As a result, the accuracy of the measurement can vary depending on the surface you run on.
Please note that generally speed and distance readings measured with an accelerometer from your wrist are not as accurate as when measured with GPS. If you suspect your speed or distance readings are flawed, check that your physical settings are correct in your device, and consider could any of the factors listed above affect the readings.