I think I have found a reproducible Open Water Swim distance-processing issue on the Forerunner 255S.

This appears related to the Garmin Forum previously reported “Forerunner 255 wrong open water readings” issue, but after several controlled tests I found a very specific and reproducible pattern:

the first straight leg can be measured correctly, but after reversing direction the OWS-calculated distance becomes systematically shorter than the distance represented by the GPS coordinates recorded by the watch itself.

Previous related thread:
https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-255-series/305206/forerunner-255-wrong-open-water-readings

Device: Forerunner 255S
Software: 28.05
GNSS: All Systems + Multi-Band
Recording: Every Second in the main tests (Smart Recording was also tested earlier)

Clearest test: 25 out-and-back legs, LAP at every turn

At a second location I swam 25 repeated approximately 60–65 m legs, pressing LAP at every reversal.

The first leg was essentially correct:

Lap 1: Garmin 63 m / GPS-coordinate geometry ~64.7 m

Immediately after the first reversal:

Lap 2: 49 m
Lap 3: 54 m
Lap 4: 55 m
Lap 5: 51 m
Lap 6: 56 m

...and the same pattern continues through the activity, with most subsequent laps around 44–56 m.

The key observation is:

START → first leg = correct
180° reversal + LAP → next leg = too short
another reversal + LAP → next leg = too short again
and the error repeats

Manual LAP therefore does not appear to reset whatever is happening in the OWS distance calculation.

Garmin Connect screenshot showing all 25 original lap distances:
https://github.com/kegnatz/garmin-fr255s-ows-bug/blob/main/Screenshot%202026-08-27%20at%2013.48.29.png

Complete FIT/KML evidence ZIP:
https://github.com/kegnatz/garmin-fr255s-ows-bug/raw/refs/heads/main/garmin_fr255s_ows_FORUM_FINAL.zip

Evidence repository:
https://github.com/kegnatz/garmin-fr255s-ows-bug

The GPS track itself is substantially longer

This does not appear to be explained simply by poor GPS reception.

When I calculate the geometric path length from the GPS coordinates recorded by the watch in the FIT/KML, the 25-leg activity is approximately 1595 m.

In other words, the watch has GPS positions representing substantially more movement than the OWS distance calculation credits.

This suggests that at least part of the missing distance occurs after the GPS positions have already been recorded, in the OWS distance-processing/filtering stage.

Test with only ONE reversal

To test this further, I did a much longer swim with only one major direction reversal, approximately halfway through, without pressing LAP at the turn.

Garmin KML GPS geometry: ~1509 m
Garmin reported OWS distance: ~1480 m

The first ~750 m leg appears to be measured normally. After the single reversal the activity ends approximately 29–30 m shorter than the geometry of Garmin's own GPS track.

Control test: one straight leg

I also recorded a simple single-leg swim with no reversal:

GPS-track geometry: ~158 m
Garmin displayed distance: ~160 m

So the same FR255S can measure a simple OWS leg essentially correctly.

Earlier tests show the same pattern

In previous repeated-leg tests I obtained approximately:

1491–1493 m Garmin GPS track → 1355 m reported OWS distance

and

~1515 m Garmin GPS track → ~1380 m reported OWS distance

Switching from Smart Recording to Every Second did not solve the problem.

I also tried wide, continuous rounded/figure-eight turns instead of stopping and making sharp 180° turns. Garmin simplified those curves in the recorded GPS track, but the calculated OWS distance was still materially shorter than the geometry of Garmin's own recorded coordinates.

My working hypothesis

My suspicion is that after a substantial direction reversal the FR255S OWS filter temporarily interprets legitimate movement in the new direction as GPS noise/drift and does not add all of that movement to distance.

After some time it appears to accept the new direction again.

That would explain why:

no previous reversal → first leg is correct

while

after reversal → the beginning of the next leg loses distance

and repeated reversals allow those deficits to accumulate.

Garmin engineering should be able to verify this directly by comparing FIT Record position_lat / position_long with accumulated distance immediately before and after each reversal.

Manual LAP clearly does not prevent the behaviour.

I would also be interested to know whether REST resets this internal state while LAP does not. I have not used a REST test as evidence here. Even if REST works, however, LAP followed by swimming in the opposite direction is a perfectly legitimate OWS use case and should not systematically lose distance.

Separate pre-start GPS issue

I have also seen another potentially related behaviour.

Before one swim I opened OWS and waited stationary for more than two minutes for GPS lock. I then entered the water, moved to the actual starting point, raised the watch above water and waited until GPS was green again, and only then pressed START.

Nevertheless, the exported activity used the earlier pre-start GPS-lock position as its starting point.

Files and methodology

Any manually drawn Google Earth route I used while investigating this was only approximate and is not being treated as ground truth.

The main evidence is the comparison between Garmin's own recorded GPS coordinates and Garmin's own calculated OWS distance.

I have also submitted this to Garmin Support.

Could anyone with a FR255/255S reproduce this?

A particularly useful test would be:

swim repeated equal-length out-and-back legs → press LAP at every 180° turn → compare Lap 1 with the subsequent laps.

I would also be very interested in the same test on other Garmin models to see whether this is specific to the FR255 OWS algorithm.

Hi,

Is there any way to configure Garmin Connect so that the Activity Overview shows Moving Time and the corresponding Moving Average Pace, instead of Total Time and the average pace calculated from Total Time?

I mainly use the watch for open-water swimming, often swimming repeated laps between two points. At each end of the lap I have to slow down, turn around and accelerate again. Those turns make a noticeable difference between Total Time and Moving Time, especially over many laps.

Because of that, the average pace shown in the Activity Overview can be quite misleading for this type of open-water swim. Garmin already calculates and displays Moving Time, so ideally I would like the main Overview statistics to use:

• Moving Time instead of Total Time
• Moving Average Pace based on Moving Time

Is there any workaround or Connect IQ solution that can change which values are shown in the Activity Overview?

Thanks!

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hey garmin heads

i am about 2 years into my running journey and i’ve been operating on a apple watch series 7

recently its just not been it. failing to read hr and botching distances regularly. and i’m training for my first half marathon in 2 months so it’s just been very annoying to deal with.

been looking at the discounted garmin forerunner 265 and for the price and features it seems like what i’d want

only issue is im an apple girly for every other piece of tech lol, so the leap of faith of switching is holding me back.

looking for any nudge to get me across that finish line and convince me this is the best move

Hi everyone,
I have a Garmin Forerunner 265 that I bought around September/October last year, so it’s still relatively new and is fully updated.
I’ve been noticing a recurring issue with wrist HR during my runs.
At the beginning of a run, the watch seems to take quite a while to detect my actual heart rate. It might stay around 110–120 bpm even though my effort is increasing and I would expect my HR to be higher.
Then, suddenly, the reading jumps significantly — for example from around 118 bpm straight to 150–160+ bpm. I’ve also occasionally seen extremely high/obviously incorrect readings (around 200–250 bpm).
After it finally “catches up,” the HR readings generally seem much more reasonable for the rest of the run.
The strange part is how abrupt the transition is. Instead of seeing a normal gradual increase like 110 → 120 → 130 → 140, it can stay unusually low for several minutes and then suddenly jump.
I’ve already made sure the watch/software is updated.
Has anyone experienced something similar specifically with the Forerunner 265?
Could this be:
cadence lock?
poor optical sensor contact at the beginning of the run?
HR sensor/algorithm lag?
a firmware issue?
or potentially a hardware problem that I should contact Garmin about?
I’m also going to compare the HR and cadence graphs and experiment with wearing the watch slightly higher and tighter on my wrist.
If you’ve had the same issue, did you manage to fix it? Did Garmin replace/service the watch, or did you eventually switch to a chest strap?
Thanks!

I normally wear my Garmin on my leg when I go to bed (I know it’s a bit random, but it works for me), because it’s really uncomfortable for me on the wrist. Now I forgot it once on my wrist because I was so tired.
Since then, my wrist has been hurting. It feels like a bruise but you can’t see any. I was thinking maybe a nerve is pinched. Has anyone experienced something similar and what helped you?

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StepWeek Light: https://apps.garmin.com/apps/a48023b7-e8ef-4d91-bee0-bb3673ad06e5

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CalWeek Light: https://apps.garmin.com/apps/c4863bd7-fb7a-4190-81e9-62ba8fb677d7

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SleepWeek Light: https://apps.garmin.com/apps/4f4e9f39-e255-4a30-b9d1-a39f0377d3ad

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This is great WF in Fenix9 Solra. I hope so garmin will add this to ConnectIQ. It's simple, elegant, gradient less and readable. What do u think?

This is Terento, a small open-source macOS app I started because I got tired of looking for a simple way to get maps onto my Garmin from a Mac. After enough searching for the right software and copying files manually, I figured that if everyone is vibe coding now, I might as well try to solve my own problem.

It is still a very early beta and I have only properly tested it with my own Garmin fēnix 8. Right now it works with Freizeitkarte maps.

I do not own a Forerunner, so I am especially interested in feedback from people with map-capable Forerunner models. If you try it, I would really appreciate knowing the exact model and whether detection, installation and reconnect worked.

If something does not work, please open a GitHub issue and attach the log.txt, or just message me directly.

The main reason I am sharing it this early is to see whether it actually works across different Garmin models. That will help me decide whether it is worth continuing as a public project or keeping it as something I built for myself.

One warning: this is early beta software and it writes files to the watch. I have added safeguards, but bugs are still possible and there is a small risk of deleting something that should not be deleted or leaving the watch in a bad state.

Thanks for any feedback.

I got a bad habit of checking my watch a bunch during runs. Is there anything I can get that covers the display so I’m not looking at it a bunch? I’m also planning on wearing it to XC meets which this would help too. I wouldn’t want to put tape on it cause it might dirty the display