Originally Posted by
ItsJustMe
Probably a valid theory, but I misspoke, I ADDED 2% to the comp distance. The GPS actually indicated a farther distance than the bike comp did, so if this did come into play, I should have added even more.
My bike comp reads to within less than a tenth what any of our three cars do for the same route, so I'm OK with it anyway.
I think riding a known distance might be more accurate than measuring rollout; tires compress and squirm a bit on pavement under load. It's probably not a lot, less than a mm or two per rev, so probably not worth worrying about.
I bet those GPS distances are based on map info which is far from perfect and 2% is pretty close. I think GPS is only accurate to a few feet so it'd be pretty difficult for it to decipher your exact travel distance without using map information.