Originally Posted by
Fiery
Oh my, I do apologize for daring to disagree with Sheldon Brown.
But do read his method more carefully - notice the "10 miles or more" part. Riding a one-mile course mapped through MapMyRide simply isn't very accurate.
"Sheldon Brown is dead" tadadada DAMDAM! tadadada DAMDAM! tadadada dadadada dadadada DAMDAM!
No seriously you and mr. Brown are right that one mile isn't enough to be very accurate, but if you use this technique from and to two easily distinguishable points on the map and following a straight road for more than 10 miles ... it will definately be accurate enough.
It cannot ever be completely accurate because there are other factors like the heat of the road that will heat up the air in your tyres pumping them up slightly and thus increasing the length of one rotation or when you go up a steep hill the pressure will make the length of a rotation smaller, etc ...