Edge 820 and Garmin Magnetless Speed Sensor

The 800 uses the GPS to calculate the wheel circumference. That unit also lets you specify the wheel circumference.

The 820 should do the same. You might have to look around in the menus to find where the circumference setting is. (Maybe, in the bike profile...)

When the sensor is paired, connected & active there is an additional setting for the circumference under the sensor in the settings menu.

Something like Settings > Sensor > {Sensor name} > wheel size

Correct. The option to set wheel size is only available when the sensor is connected and active. I thought this was pretty weird when I discovered it but that's the way it is. I had to go out the garage with the 820 and turn the crank on my bike to wake up the sensor in order to be able to set circumference.

Thanks looigi and The_Hunnymonster - that's the missing link in the set up; the sensor has to be active and connected. Then a sub menu miraculously appears in the Sensor settings allowing you to manually set the wheel circumference. It also tells you what the current "auto" circumference is.

Out of interest, my "auto" circumference read 2180, against my actual value of 2214. So really quite accurate on the auto setting (approx 1.5% error) and not worth worrying about (for the nerds like me, that's roughly an error of 25m over a statute mile. Apologies for mixed units, just the way my brain looks at things...)

I figure the auto circumference will be more accurate if it's calculated over a relatively straight course rather than a tortuous one. What I've done for my mtb is to allow they auto circumference to calculate over a straight course, make a note of it, then enter it manually and use that so that it doesn't subsequently change.

One thing I've done on my mtb: The tire has a large circumference and I'm often going pretty slowly so I put two magnets 180 degrees apart on the wheel to double the frequency of ticks the 820 sees. Of course the resultant auto circumference is 1/2 the actual so that the speed and distance is correct. This will help with auto pause if you choose to use it.

Remember too that your rear wheel will always follow a shorter course than the front on all but a perfectly straight course (with no drifting from side to side as you pedal). If your GPS is over the front wheel & sensor on the rear wheel that can add up particularly on a twisty course...

@looigi - the sensor I'm using is "magnetless" and uses some kind of internal "accelerometer" to, presumably, count the wheel revolutions, but I get the idea of what you are suggesting.

@The_Hunnymonster - this sensor is designed to sit attached to the hub, so it's natural position is the front wheel hub, but again, I understand what you are saying re the difference in travel between front and rear wheels.

You might think the front is natural - those of us who had combined sensors in the past would probably tend to place them on the rear wheel (as will anyone using a turbo trainer)

Out of interest, my "auto" circumference read 2180, against my actual value of 2214. So really quite accurate on the auto setting (approx 1.5% error) and not worth worrying about (for the nerds like me, that's roughly an error of 25m over a statute mile. Apologies for mixed units, just the way my brain looks at things...)

The auto setting is updated throughout the ride. So there is no way to know for how much of the ride it was using that setting. On average I think the auto setting will be closer than 1.5%.

The auto setting is updated throughout the ride. So there is no way to know for how much of the ride it was using that setting. On average I think the auto setting will be closer than 1.5%.

I don't think it is. It certainly wasn't on my 705 and 500 units, and the 820 seems to behave the same way. Auto wheel size determination is triggered by changing sensors, and some other events. One it's determined, it doesn't change until a triggering event. Change sensors, go for a ride, and it will use the first 1/2 km or so to determine wheel size and then stick with that until the next triggering event, after which it reports the wheel size has been determined.