Measuring the diameter of the wheel+tire doesn't work as well because this is a much more difficult measurement to make. You would need to measure the "effective diameter", which is the diameter of the wheel+tire when properly inflated and weighted by your bike and body. Obviously you can't do this by yourself, and when done by someone else "parallax error" can come into play in which the measurement is off due to sighting errors--the measurement could easily be off by 0.5 to 1cm in either direction. This alone would be about 5-10 times more error than what you would get by measuring the "effective circumference" of the wheel directly by measuring the distance the bike travels in one wheel revolution, and then you have to multiply the measurement by Pi which would multiply the error by 3.14-fold, for a total error increase of about 16 to 32mm in the calibration number, which would work out to about 40 to 80 feet per mile.