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Instant pace is not accurate and has bias towards slower than actual pace

It appears instant pace on my Fenix 6X Sapphire has a consistent bias towards a slower than actual pace, often by 0:30-1:00 min/mile. 
That makes it more difficult to pace accurately, for example during races. It seems the bias is worse on more challenging terrain, for example on trails or under tree cover.

To understand this better I wrote a python script that parses a run activity that I export in TCX format (for easier parsing because TCX is a text based format).

Here are some examples of my script output. This is from a faster paced trail run on moderate tree covered trails:

Mile 1.00: Split: 8:42, Avg Pace: 10:04
Mile 2.00: Split: 8:52, Avg Pace: 9:22
Mile 3.00: Split: 8:37, Avg Pace: 9:17
Mile 4.00: Split: 8:04, Avg Pace: 8:31
Mile 4.53: Split: 8:03, Avg Pace: 8:03
----------
Overall pace 8:30, Avg Pace: 9:08

In this example Split time is produced every mile based on elapsed time from the beginning.
Avg Pace is produced by looking at the instant speed reported each second in each sample, averaging it over all samples of that split, and then converting that average speed to pace format (in minutes per mile). Basically Avg Pace represents the averaged result of what the watch was showing me during the run.

If anyone questions that approach, it should be OK to average the speed because it is sampled at even intervals every second (it wouldn't be OK in the case of smart recording).

As you can see there is quite a bit of discrepancy, especially in the beginning, although it gets better towards the end. Overall, after averaging, the watch reported 0:38/mile slower instant pace than what I actually ran, so there is a strong bias towards slower pace.

Here is another example - this is from a mix of road and suburban trails on more open terrain:

Mile 1.00: Split: 9:07, Avg Pace: 9:22
Mile 2.00: Split: 8:04, Avg Pace: 8:09
Mile 3.00: Split: 10:49, Avg Pace: 10:43
Mile 4.00: Split: 10:35, Avg Pace: 11:30
Mile 5.00: Split: 8:23, Avg Pace: 8:18
Mile 6.00: Split: 13:05, Avg Pace: 13:39
Mile 7.00: Split: 7:58, Avg Pace: 7:56
Mile 8.00: Split: 9:08, Avg Pace: 9:34
Mile 9.00: Split: 8:11, Avg Pace: 8:44
Mile 10.00: Split: 8:43, Avg Pace: 8:46
Mile 11.00: Split: 10:08, Avg Pace: 10:13
Mile 12.00: Split: 8:22, Avg Pace: 8:32
Mile 13.00: Split: 8:29, Avg Pace: 8:30
Mile 13.76: Split: 10:02, Avg Pace: 10:00
----------
Overall pace: 9:21, Avg Pace: 9:32

Even though this is much better overall, during some miles the discrepancy between the split times and the averaged instant pace was still up to 1 min/mile.

One more example - this is from a much slower mountainous trail run on steep terrain with a good amount of walking:
Mile 1.00: Split: 9:44, Avg Pace: 10:17
Mile 2.00: Split: 11:44, Avg Pace: 12:15
Mile 3.00: Split: 14:14, Avg Pace: 14:08
Mile 4.00: Split: 29:14, Avg Pace: 27:51
Mile 5.00: Split: 17:40, Avg Pace: 20:02
Mile 6.00: Split: 12:23, Avg Pace: 12:43
Mile 7.00: Split: 12:36, Avg Pace: 13:46
Mile 8.00: Split: 11:53, Avg Pace: 12:34
Mile 9.00: Split: 14:34, Avg Pace: 15:07
Mile 10.00: Split: 24:11, Avg Pace: 23:23
Mile 11.00: Split: 8:50, Avg Pace: 8:46
Mile 12.00: Split: 12:23, Avg Pace: 13:31
Mile 13.00: Split: 10:46, Avg Pace: 11:50
Mile 14.00: Split: 16:09, Avg Pace: 16:42
Mile 15.00: Split: 17:20, Avg Pace: 17:51
Mile 16.00: Split: 13:23, Avg Pace: 13:32
Mile 16.68: Split: 11:14, Avg Pace: 12:32
----------
Overall pace: 14:40, Avg Pace: 15:15

In this case the instant pace was faster than actual in a couple of splits, mostly in very slow ones where I walked or stopped. But the overall pattern is the same - there is a clear bias towards a slower pace.

I should add that today I installed a Rolling Average Pace Garmin IQ field that averages pace over the last 100 yards. I placed that field next to Garmin's Instant Pace and watched them side by side during an easy run. One thing was clear, every time I reached a steady pace and cruised for a while to let the rolling pace stabilize, the rolling average pace was always a bit faster than Garmin's Instant pace, which confirmed the same bias that I discovered from the post-analysis of the runs with my script.

Has anyone had similar observations?

12/05/21 EDIT: I changed the title of the post since Garmin seems to have improved the pace. It is more stable and precise than before, meaning that the values are closer together, but it is still not accurate - there is still a significant bias towards slower than actual pace

  • Yes, I found info of vertical ratio % and its calculated by step length and vertical oscillation.

    For your information I have registered 2 cases regarding this pace issue and also about the GPS data field which shows wrong GPS signal strength. Haven't received any answers yet but I can keep you updated later on. 

    I will continue with my calculations when I have time. I want to reproduce the algorithm for Garmins way of calculation the Pace but I'm not there yet. I'm relative close but not 100% accurate.

    Have you looked at the data for gps_metadata.enhanced_speed in the .FIT files? That one looks really off for me.

  • This post is roughly a year old, but the problem is still the same. Any news on the horizon for a future fix...?

  • Total silence from Garmin about this issue...

  • I own a fenix 6 and I have the exact same issues that all of you guys have. At first I didn't mind that much and I lost the golden opportunity to return that watch to Amazon with no questions asked. So now that I am trying to follow a specific training plan for an upcoming marathon I am very frustrated as it is impossible to obtain any useful pace information from this watch during the run. I have recently contacted garmin support about this. Needless to say, the representative responds like she have never heard about this before. Her suggestion so far was pretty useless:

    1. Wait for the GPS Ready message or GPS Indicator to turn green
    2. Wait 2-5 minutes before you start the timer for your activity

    I responded that I am well aware of this and I always try to give my watch the extra time to sync with more satellites. Today she came back with another suggestion, connect to PC through USB, locate and delete the RemoteSW folder. I think she just wants the EPO files to be refreshed, and I am 99% sure that this won't work, but I am going to try this anyway on my next run.

    I let her know about the big number of unsatisfied fenix 6 users by showing her this and some other threads, I got no comment from her.

    Has anyone seen a big improvement after using the foot pod?

  • Here is my update 2 years later - the issue is still pretty much present and Garmin seems to be either ignorant or unable, or unwilling to fix it.

    Below is a speed graph derived from a TCX file exported from my yesterday's run. That was a moderate 7.5 mile trail run in a local watershed preserve. The tree coverage was heavier in the first 45 minutes, then less heavy until about 57-58 minutes, then mostly open the end. The trails are fairly straight - there are no switchbacks and not many sharp turns. There are some mild rolling hills, but nothing major. The entire run was only about 500 ft of elevation gain.

    On the Y-axis is speed in meters per second. For comparison 2 m/s is about 13:25/mile, 3 m/s is about 8:55/mile, 4 m/s is about 6:40/mile

    The blue line shows the Fenix 6X speed as reported by device during activity and stored in the file. The red line shows 60-sec rolling average speed based on GPS coordinates only. The grey lines show instant 1 sec GPS speed, which is basically distance in meters between consecutive GPS points. I used industry standard haversine formula to calculate the GPS distance.

    As you can see there is still a pretty significant bias towards the slower speed (note that I use speed interchangeably with pace here because that is basically the same thing). Especially in a denser wooded areas the difference is quite substantial and at some points reaches 3-4 min/mile. On the open towards the end the device speed is reasonable accurate - no complains there.

    I should also mention that at the end the device distance was 0.35 miles shorter than the distance derived from GPS points using haversine formula - 7.21 miles vs 7.56 miles. The latter is the same as the distance shown in Strava after distance correction.

  • I went for my first run using the Fenix 6 a few days ago. I had no idea about the GPS/pacing issues with this device, but the instant pace inaccuracy was so severe that I practically gave myself a heart attack trying to stay within my target pace. It would stay unreasonably high given my HR, and then it would suddenly drop by more than a minute within a span of 5 seconds. To add insult to injury, my distance calculation was underestimated by about 5% so my splits were much worse than usual despite almost killing myself. I'm glad I didn't sell my FR245 yet. It's so disappointing that it seems like this issue will never be solved.

  • I spoke with an ex employee of Garmin customer service, now working in my company. GARMIN know very well about this issue, which happened when they shifted to a different model of chip for GPS and other processing. However, they are not going to acknowledge it publicly (they won't) or do anything about it for existing models (they can't). They seem to consider customers will still stick with the brand despite that, and they'll fix it with the next generation 

  • However, they are not going to acknowledge it publicly (they won't) or do anything about it for existing models (they can't). They seem to consider customers will still stick with the brand despite that, and they'll fix it with the next generation 

    This is very sad to hear and doesn't speak highly about Garmin. How can Garmin expect users to remain loyal? 

    Perhaps it is not a surprise that I see a similar deniability pattern with other Garmin devices, for example Garmin Index S2 scale. For anyone curious, you can check Index S2 forum to see what I mean.

  • . To add insult to injury, my distance calculation was underestimated by about 5% so my splits were much worse than usual despite almost killing myself. I'm glad I didn't sell my FR245 yet.

    See what happens if you run with both at the same time - I did this back in March this year and generally they matched up very closely, although I did see one instance where the F6 Pro came up shorter than the 245 distance wise for no obvious reasons I could discern. My results with 245 / F6 are in the long standing 'running distance too short' thread.

    I do not seem to see the short distance issue on measured courses during measured and certified races, often running with a second device also recording - however just because I do not see this issue myself, the evidence that @SilentVoyager and @AndersB and others have presented is quite compelling.

  • The distance issue seems to be complicated. On one hand distance seems to added due to wobbling, but that has more effect at slower speeds and on straight stretches. On the other hand distance is cut in turns. The higher speed is the more watch tends to cut through corners. But also I see distance being cut even on straight paths when there are issues with GPS signal strength which seems to make Fenix prioritize accelerometer data over GPS data. I've seen some evidence of that.

    Furthermore to make things even more complicated I start to suspect that the size of watch makes the difference too. That is because the bezel is used as GPS antenna, but the antenna size makes a difference when it comes to sensitivity. An ideal antenna size should be closer to 1/2 or 1/4 of the wavelength. GPS L1 wavelength is 19.05 cm.

    Based on what I've seen in the forum, Fenix 6s users seem to be most happy with the accuracy and Fenix 6X - least happy. Other smaller Garmin watches seem to be more accurate too. But this is entirely my speculation. 

    Regarding accurate distance reporting of Fenix in races, it is worth considering that in big races most runners end up running longer than official distances due to not taking the most optimal path. Race distances are measured by taking the shortest possible path which requires running tangents and always making turns on the inner side of a turn. Most runners neglect to run that optimally and end up running some extra. So if someone gets an accurate official distance in a race, there is a good chance that their watch is actually too short. In trail races my 6X is always too short - the faster race pace is, the shorter the device distance compared to the official distance.