MTB Wheel size, speed sensor and distance traveled.

cwoodffr said:

Questions....does the speed sensor and data take over on distance calcs or is it still done by GPS?

Yes. If you have autocalibrate on, eventually it will get where it needs to be but that calibration is way off. I would delete the sensor and let the fenix autocalibrate. If you want to start (without measuring the distance traveled via one rotation) you can use this chart 

https://www8.garmin.com/manuals/webhelp/edge520/EN-US/GUID-DB0BC20A-DADF-41EB-A61C-14BEC158B43C.html

Having wheel size on auto means that the Garmin will attempt to calculate the size automatically by comparing GPS to the sensor. Setting it manually, it should be the circumference including the tire so 737 is not the correct size. This page has a table you can use to determine the circumference based on your wheel and tire size. 
https://support.garmin.com/en-US/?faq=aXLw3909O4031Yv7zjcw47

I believe that when it is paired it will use the speed sensor as the primary source, but I only use mine on my indoor bike on the trainer so I cannot answer that for sure. It will definitely not go back and correct the speed for past rides. 
Do you have the sensor on the front or rear wheel? I have found in the past that the new speed sensor works better on the front wheel. On the rear wheel I had a lot of speed issues that showed up as a very choppy speed graph in Garmin Connect. I would expect that to show a shorter distance for you than your friend, but the auto calibration would have also had issues so who knows. If you have it on the rear, try switching to the front hub instead. 

Using true wheel size will be more accurate on speed and distance, specially MTB, so many tree an woods blocking the GPS signal!

Depending on how many switchbacks were involved, I am suspecting the infamous Apple swoosh.

Explain please. 

Delete the sensor from the watch, then add it back again. Then start a cycling activity, and ride down a road without buildings or tree cover, in a straight line. Ride about one km. That will allow the watch to accurately set the wheel size using the GPS.

Ah clearly my assumption that wheel size was the diameter and it would calc the circumference was incorrect!  But still, auto would be somewhat close I would imagine.  I used the table to get the size.  I would like to know what the reference to the Apple accuracy as I always need fresh fodder for Apple criticism with my Apple obsessed friend.

hi,

I have speed2 with auto calibration.

my 29er on auto give me right now 2295mm. I have 2.3 tires underinflated at 2atm (tubeless mounted) so make sens.

it is about the circumference not the diameter.

737mm is the diameter 29"x25.4

yes, I recommend the same, use auto - will take into account the real circumference (depending on the pressure you use)

till calibrate, at least at the beginning yes would be better to have GPS....just be sure you have green GPS when start.

just me,

robert

So I deleted the speed sensor for now to eliminate.  Still coming up quite a bit longer than apple watch but when I look at the tracks in Strava the Fenix track is much more believable related to the known trail path compared to the apple....so I am telling my apple bigot friend to f off....My frustration is that he points to tests that show how the apple is top drawer on accuracy....I finally posted the image of the plotted paths blind and asked the riding group to say which one looked more accurate related to our twisty trails....all picked the Garmin of course.

Your Garmin with a speed sensor will be way more accurate than an AW with just GPS. The Garmin with a speed sensor is similarly more accurate than a Garmin with just GPS. All GPS software rounds off tight corners - how much depends on the device and the specific software, but they all do it. The speed sensor, if properly calibrated, measures how far you actually travel. The Garmin with speed sensor will always show a longer distance because of that, but it is a way more accurate distance IMO.