Garmin 1040 inaccurate speed reading

pdj-84 said:

I also have issues on my other bike without a speed sensor.

Could you post a link to one of such rides? Perhaps we can find some clues in the background data. Which GNSS settings do you use on your Edge?

Thanks! I’ll give that a try tomorrow

To the people saying "Use a speed sensor" - not helpful.  How about Garmin fix whatever is causing this.  My 1040 used to be OK with GPS speed and now its doing the same thing others have reported - showing a very low speed when there are a few trees around.  1030 Never did this and I'm pretty sure the 1040 didn't do it until recently.  Looks like another Garmin software bug to me.

The course is found here: https://connect.garmin.com/modern/course/335977703

I noticed that my position appeared to be incorrect during the "climb," showing on the car lanes instead of the cycle path, even though my speed seemed reasonable during that segment.

During the descent, my position was accurate, but my speed was less than OK (showing only 20km/h), possibly because it was trying to catch up as speed is derived from GPS position?

This happened on the segment of my route adjacent to the "Zeisterweg," right at the end of the descent of the first "climb" when reviewing the course listed above.

Below a screenshot of my altitude and speed profile:

I'd expect this sort of coverage not too affect GPS position by this much from a Multi-GNSS multi-band device.

Does Garmin-Clayton have any relevant insights to share regarding this matter?

pdj-84 said:

Anyway, the course is here: connect.garmin.com/.../335977703↷

That's just the course, which cannot help with any data analysis. The link to the activity is needed for that.

I likely misunderstood your request. However, this should point you to the activity: https://connect.garmin.com/modern/activity/18407430002

pdj-84 said:

I noticed that my position appeared to be incorrect during the "climb," showing on the car lanes instead of the cycle path, even though my speed seemed reasonable during that segment.

That small deviation is well within the guaranteed accuracy range, and cannot cause any noticeable difference of speed. 

pdj-84 said:

During the descent, my position was accurate, but my speed was less than OK (showing only 20km/h), possibly because it was trying to catch up as speed is derived from GPS position?

The activity data shows you used a speed sensor during this ride, so GPS is not used for the distance in such case.

The data shows the speed of over 40 km/h during the descent (in red), and then it drops to some 22 km/h in the middle of the descent (dark blue). The GPS track is all right, as you told yourself too, so that's not an issue here (even if there were no speed sensor used). Did not you simply start braking on the slope as you were approaching to the road crossing? Also, I see there were multiple sensors connected, including a cadence and shifting sensors, so the question is whether the speed value (if really not corresponding to the reality) was not somehow the result of improper processing or incorrect setup of the sensors. For example if the drop of the speed occurred in the moment you shifted the gears, or when you stopped pedaling, there might be a problem in processing the data from the sensors, or due to some interference. Is the sensor sitting firmly on the hub, without sliding? 

Thank you for your analysis. To clarify, I wasn't braking at that time, and I did notice the lower speed while descending and I thought to myself that something seemed off. I have a speed sensor, di2, HRM, and power/cadence sensor connected. The lower speed reading appeared briefly for several (up to 30s), and I don't believe it was just a minor or temporary issue.

As I mentioned earlier, I have observed similar behaviors on my other bike that doesn't have the speed sensor. While it’s not as pronounced as in this instance, there are times when I am pedaling comfortably at around 30 km/h, and my speed unexpectedly drops to 24 km/h, which I don't attribute to wind gusts or changes in cadence. Unfortunately, I don't recall when exactly so cannot share the activity at this point.

pdj-84 said:

Unfortunately, I don't recall when exactly so cannot share the activity at this point.

A link to such activity could be important to pinpoint the culprit. Try finding it, or reproduce anew. And if you are going to contact the Support for reporting the issue, it is important to provide as much evidence as you can, since otherwise they will simply give you a canned answer, or tell to reset the device (which might be indeed worth of testing anyway).

BTW, did you use any other sensors on that ride too (except of the HRM)?

Yeah I know, I will keep an eye out for when it happens with the other bike as well. On that other bike I have a power/cadence sensor (PM in crank) and HRM only. Besides the Garmin sensors.