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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

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0 chrisdr2 over 4 years ago

Instant pace does swing with tough GPS conditions. Using a foot pod and setting it to speed=always, may help with that.

0 silentvoyager over 4 years ago in reply to chrisdr2

It isn't the point that it swings. That is obvious. The point is that is has a consistent bias into the slow side. I don't want to have yet another device attached to my shoes because I rotate through a lot of shoes. Suunto manages to have a far more accurate instant pace using FusedSpeed technology. Garmin could manage the same by tweaking the algorithms a bit. A more accurate instant pace could be achieved by fusing input from the watch accelerometer (cadence) and I think Garmin already does that to some degree, but the algorithm does need further tuning.

Edit: the reason I think Garmin already fuses accelerometer input is because the speed quickly drops to zero (--:-- pace) once I stop moving. If that wasn't the case it would continue having a small speed because the GPS position would continue to randomly move around my current position. Also Garmin watches are capable of measuring speed even without GPS by using only wrist movements.

0 chrisdr2 over 4 years ago in reply to silentvoyager

Maybe they use the accelerometer as a trigger to ignore GPS when stationary.

0 C.sco over 4 years ago in reply to chrisdr2

The accelerator probably does a combination of both. Detects stops, but also helps estimate speed and heading. I walked through a very long train tunnel once, and the track on the map continued to plot itself as I walked through the tunnel, albeit in slightly the wrong heading (but fairly close). It also showed my speed. When I got to the other end of the tunnel and regained GPS, it self-corrected the tunnel section with the slightly improperly plotted heading, and continued on as if nothing happened - it did that with accelerometer only, as there's no way I had any GPS under 500ft of earth.

0 chrisdr2 over 4 years ago in reply to silentvoyager

As far as I am aware, Garmin used a new GPS chipset for the latest range of watches, including F6. It's possible that there are some differences in how they perform in challenging environments - suggest that you send your data to Garmin support, see what they say.

0 scotthunter2 over 4 years ago

The only 4 data fields you need on your watch are timer, distance, lap pace and average pace.

0 silentvoyager over 4 years ago in reply to scotthunter2

You shouldn't make any assumptions. I run on trails where terrain is highly variable. Lap pace has very little value because within one mile my pace can sometimes vary from 6 min/mile to 60 min/mile (I am not joking). Knowing an average pace for entire mile has very little value when running on trails. At the same time vertical metrics are quite important.

If this watch has pace field there is no reason why it shouldn't be more accurate. There is a method for fusing multiple sources of distance and speed that complement each other, that together produce higher accuracy than each other separately, and that isn't a rocket science. Producing a more accurate instant pace is certainly possible with a relatively simple algorithm. I am going to contact support and see what they think about this.

0 Antoine B. over 4 years ago

Yes, during runs the instant speed or pace are often wrong (and much lower than reality). I run the same courses all the time in my neighborhood (low wooden houses mostly, so no GPS challenge) and I am getting wrong readings although the overall average pace seems OK.

I just changed from a 645 to a Fenix 6s and the 645 had absolutely no issue on the same courses just a couple of weeks ago.

Very bad impression... Hopefully Garmin people develop a fix asap!

0 Klick over 4 years ago in reply to silentvoyager

I've got exactly the same problem with a Stryd, I was even thinking to dream sometimes, and wrongly concluded it was a problem with the 'always" setting, ie, the watch using the internal speed instant of the instant one... but it's not the case :

What kind of pattern is extremely common for me :

- Running at x+y (instant pace, given by stryd, really accurate, configured the garmin to always use the stryd)

- my total pace will be set at X

- error factor being Y, for the whole duration.

in which universe when you run slower (instant pace) than something, is your average speed HIGHER !

broken? yes realy ;)

0 diaks over 4 years ago

Yes, i have same impression. Instant peace is almost always below average peace from one km. I am running around 5 min/km and watch is showing between 5:30 to 6:30 min/km. It worse in forest and when route is not straight.