I'm new to all this fancy gps computer stuff. I have a Garmin 800 and I see that I can either manually set the wheel or have it automatically determine it. I do a fair bit of riding on my rollers in my basement, so I figured I needed to manually set the circumference. Does anyone know if that is correct? Or if I have it set to automatically determine it, and it does so for a long ride, will it "remember" that circumference for the next 4-5 rides I do in my basement on the rollers before I get out on the road again? I mean I could of course just get some thread out and measure the actual circumference of the wheel, but I'm lazy and figured that the garmin could calculate it well enough if I go on long enough rides (though asking it to remember whatever it set last rather than try to recalculate when it can't find a satelite might be a bit much).

Yes, you would need to set the circumference manually if riding in the basement and if you put a different sized set of wheels on. I cannot see any means by which the Garmin can calculate the wheels circumfernce unless it has both the cadence sensor with the rear wheel magnet AND the GPS signal, which in my basement ain't happening. I am sure my little stalker will correct me.

If I always use the same wheels, I wonder if it'd just stick with what it calculated last or if it automatically goes to a default.

I've found the mileage totals are always low if you let Garmin Edge devices auto calculate. I always manually enter my wheel size.