Apple Watch Indoor Run

Calibrating your Apple Watch for improved Workout and Activity accuracy

That's not something you can 'fix' per say. Both devices (Nordic Trac and Apple Watch) are estimating the distance as you're really not traveling at all (I'm candidly surprised the Watch estimates anything.) The .1 difference is acceptable to me given an indoor treadmill activity. You'll get deviations in heart rate, calories burned, etc between any health products as they are making calculations and estimates all the time.

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What makes you think the treadmill is accurate,in my local gym you can't get the same result from any two machines when two people run for the same amount of time.

You're certainly right on that! I never said I thought the Treadmill was accurate, but I do think it's accurate enough for an indoor run. Same with the Apple Watch.