Watch recording different mileages

I’ve been doing longer runs with a friend recently and we both have the same watch, bought at the same time. Neither of us have auto pause on. The longer we run the more difference gets larger, hers recording up to a mile more than mine if we get up to 20 miles.
Even though we have ran the whole thing together, the pace/elevation/mileage/moving pace/elapsed pace is always completely different.
Any ideas?

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trux 10 months ago +1

The watch uses a combination of GPS and accelerometer data to estimate the distance. And it does it while relying relatively a lot (too much on my mind) on the accelerometer data, even when the GPS track…

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0 Krum 11 months ago

Maybe one of you has 3D distance and 3D speed turned on?

0 Krum 11 months ago

Maybe one of you has 3D distance and 3D speed turned on?

0 AgentMax 11 months ago

Do you use HRM Pro (Plus)?

0 LisaEdwards7719 11 months ago in reply to AgentMax

No I don’t have that on

0 LisaEdwards7719 11 months ago in reply to Krum

I’ll check this

0 trux 10 months ago

The watch uses a combination of GPS and accelerometer data to estimate the distance. And it does it while relying relatively a lot (too much on my mind) on the accelerometer data, even when the GPS track is reasonable all right. It works well, as long as the watch is already well calibrated with previous runs, but it can also result in differences up to 10% in some cases (exceptionally perhaps even more). Resetting the watch and doing a couple of runs in good GPS conditions on a flat open terrain may help reestablishing the proper calibration. Another good workaround is using the HRM-Pro strap that enhances the accuracy greatly (as long as you keep it in the default settings).

You can also use the function "Save as Course" in the activity menu on GC Web - the Course then shows the pure GPS distance, which in most cases is much more accurate. And if your GC account is linked to Strava, you can use their function "Correct Distance", and the distance will be recalculated using mostly the GPS data. Usually it works even better than the saving as Course in GC. Once you get the corrected distance, you can edit the activity in GC, and correct the distance accordingly.