I don't think Garmin has ever said. To avoid surprises that have happened to me in the past, I now always set the wheel size manually. That way you're never in doubt.
It depends on the unit. Older units did a one time wheel size calculation. Newer units use a moving average to adjust the size if needed. If you know the size and are not switching the unit between bikes I would just set it manually. One less thing to go wrong.
Even if you are switching the unit between bikes, newer models keep the wheel size attached to a speed sensor id, while older models keep the wheel size associated with a bike profile. So you can have multiple manual wheel sizes provided you're not also swapping a sensor between bikes but have a separate sensor for each bike (as I do).
So, I know this is an old-ish thread, but does anyone know how to turn off the notifications that wheel size is set? I have a gsc-10, and a new 645 watch, and it was constantly alerting me today on my first ride with this watch that the wheel size had been computed/set. Its nice that it was doing it, but I don't need it doing it what seemed like 50 times an hour.
