GPS accuracy

Did you both start and stop your devices at the same time?

First of all I never use GLONASS anymore as I have hardly ever seen it improving things...

Second, without a track it's hard to say anything. Also, what does garmin connect say about distance... Strava uses it's own/extra smoothing so might also influence distance.

How long before starting did you get a fix? It is best to get a fix a few minutes before or so, green isn't necessarily great accuracy right away.

Ok, I'll turn GLONASS off.
I had a fix long before starting, I used my watch with the warming up.

I have added 2 screens from garmin connect, the running contest was 2 rounds, the second round isn't accurate after the start it goes way a different track on the map but the 2 round must be identical.

ciq.forums.garmin.com/.../1277236.png

This is a export off the fit file (transferred to a csv file), their are a lot off errors in it, errors like this: [TABLE="border: 0, cellpadding: 0, cellspacing: 0, width: 48"]
[TR]
[TD="width: 64"]Error at pos 0BBF:1401400EDB01D601D001FFFFFFFF0D30E16A34919E2A2527B2FF045A[/TD]
[/TR]
[TR]
[TD]data,12,0,2635,4768,22,3,153,12,38,7,44,2[/TD]
[/TR]
[TR]
[TD]Error at pos 0BEA:6E12263900002C01816A1401400ECE01CA01FFFFFFFFFFFF0D31E16A349A9C2A2568B0FF04700E00004B0A03143403540B860622028412143C00002C0184661401000EC201BD01FFFFFFFFFFFF0D32E16A34A39A2A25B5AEFF04831000004B0A11154503540B9A0616029D124E3E00002C0186651401400EBC01BB01FFFFFFFFFFFF0D33E16A34AF982A2531ADFF04871200004C0A11154503540BA4060D029D12C63E00002C018A651401400EB101AE01B001FFFFFFFF0D34E16A34D3962A2564ABFF048B1400004C0AD41359039F0BD6060D02A7128A3E00002C018B651401400EA901FFFFFFFFFFFFFFFF0D35E16A340B952A2589A9FF04841600004C0AA5134F0303[/TD]
[/TR]
[/TABLE]

The distance was 5K,

Just because the organizer declared it as 5k that must not be true. Some organisers round 4.x k generous to 5k. I also experienced bigger offical 10k that were wrong measured. So your measured 4.6k might be correct! What says track as layer on satelit photos?

in the woods with many turns

Difficult for any consumer GPS to accurately measure distance and track course under those conditions.

Strava uses a different method to compute distance from recorded data. How about posting a link to the activity from Garmin Connect?

I just did a 25 mile bike ride (GPS only on 935). The distance for the 935 and my Garmin Edge 1000 was exactly the same.

I just looked up FlyBys (in Strava) for that run. Nearly all people recorded either 4,6 or 4,7km (this also includes an Ambit2 and a Polar which are known to be more accurate). So the device that we have to question is the 230 of your co-runner that summed up 4,9k.

I went from a 310XT to a 935, and I too find the accuracy of the 935, on trails, to lacking. In an open area, the accuracy is very good, but anytime there is any type of a canopy, the accuracy appears to be diminished. I pretty much figured that I lose 1/10th for every 2 miles I run on trails, sometimes more. Even though they tout the 935 as being the most accurate, it comes nowhere close to the accuracy of my 310XT. I'm not sure what the 310 has that the 935 doesn't, but on trails, there is no comparison.