Speed sensor accuracy (magnetless)

It would be worth trying a manual setting. For a 25c wheel, the setting should be close to 2105. The distance being too short and the wheel size setting too big leads to the possibility that the Forerunner is not getting the correct number of sensor triggers. There is information in the FIT file that might help if you care to either post the file or pm a link for the file to me.

thanks for your response!
Below is the .fit file for one of this workouts. I tried setting the wheel size to 2105 but got the same results.

The file link doesn't work.

I sent the file through a PM. Thanks!

The reception rate for the sensor is 95% which is good. It seems the automatic setting is erratic. I'm not sure why that would be. It appears to be a software problem. I recommend using a manual setting of 2105 for the 25cc tire to see how that works. The attached file has the sensor data replaced with GPS distance.

Thanks guys for your support.... the Forerunner 920xt SW is 9.7 and GPS SW is 3.30. GPS has Glonass off right now. The speed sensor does not seems to be loose, but I can take a look to see if at some moment it gets loose.
Also, I set manually the wheel size to 2105 but get almost the same difference vs GPS. Today I compared with a friend, and his sensor automatic wheel size detected is 2264 mm, for the same a 700x25c wheel.

Can you send me the file for the ride with the manual setting? How do your friend's distances come out? It's strange that both of you are getting incorrect automatic settings. Is this always on the same circuit? How do the distances come out on rides that are not on this circuit? The speed sensor senses the earth's magnetic field and I wonder if there might be some magnet field on the circuit that is messing up the sensor values. Another possibility is a magnetized chain that would cause problems if the sensor is on the back wheel.

Hello, I'm downloaded the FitFileRepair software, just to have a look at the data. As OnlyTwo pointed, after 6' the data starts to be different, I took the position and it is at the moment that I reach an area that goes through a road that goes over the beach. Then I kept looking, and everytime that I'm over the road that goes over the sea the difference between GPS and Sensor starts to increase, then when I reach ground the difference remains unchanged.
tigermountrider I will get the fit files from different rides in different areas to compare.


here is a pic of the area where I usually ride during the week so you can have a better idea of what I'm referring to:

Yeah, Ricardio that's pretty much what I wanted to look at with a fixed manual setting. If you are seeing that consistent pattern it looks like something about the road over the sea is creating a problem for the sensor. The solution would be to disable the sensor or use one with a wheel magnet. Garmin's magnetless speed sensor senses the earth's magnetic field. I don't know if other brands do the same or maybe they use an accelerometer.