GPS tracking

Yes, we often stand waiting for one or other of us to turn green. I just find it surprising that we have this discrepancy every day. We've both had watches for several years so we're not newbies

Compare the logged tracks and see where the difference occurs.

I'm not sure how we would do that. We're aware while we're walking that my km clock up faster than his. Sometimes it's a small discrepancy but often we start with a hundred metres but each km it grows (not by the same amount) I understand I take smaller steps so expect my step count to be higher but not distance

If I would do it I would import the other person's activity into my account and then use the compare function in the activities page: https://connect.garmin.com/modern/activities

Remember to delete the imported activity when you are done comparing. 

That the difference sometimes starts at 100m before you've even started suggests to me this is an accuracy issue between the 2 devices.

That said, it doesn't mean the devices have a problem. The accuracy of consumer GNSS ("GPS") receivers isn't superb, particularly fitness watches. Often the measured distance is off by 5-10%, and sometimes more. Usually they under-measure the true distance.

This recent research testing gives more details https://mhealth.jmir.org/2020/6/e17118/

Some factors that can increase measurement errors include things like buildings and trees, activity type (running seems to be worse than walking or cycling), arm placement, weather, and even time of day.

In your case of simultaneous activity many of those cancel out, so it may be differences in the devices themselves, your wearing position, and things like arm swing (amount and cadence).

Unless you are routinely getting differences greater than 10% between them, especially in open areas with no obstructions, then I'd say there really isn't anything unusual here. If it's worse than that though, you may want to look into it more.

Hope that helps!

Thank you, that makes sense. It's not a huge issue but rather a source of amusement between us. Most of our activity is around the coast or up the mountains on a Greek island - definitely not surrounded by lots of buildings and very few trees, just scrubland. 

At least I'm always winning

I have a Vivoactive 4s and my husband has a Vivoactive 3. We generally walk between 5 & 10 km daily so not really long walks. We're going to really monitor it tomorrow to see where the difference starts etc. It is just odd that I always complete the 1st km first and the gap between us grows with each km (although not by the same amount for each km)

It sounds then like the difference between devices is 5-10%. That could be a little high. Maybe something to try, measure one of your routes using the map features in Garmin Connect, Strava, Google Maps, etc to see what the "true" distance is. (You could either draw the route manually or upload an existing activity to see the distance.) That gives you something to compare the watch distances with so you can see how far they are off.