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the "other source" is for a finite tube raked at bicycle tube angles in the practical bicycle flow field.
No this was just from places like the one linked above http://airfoiltools.com/search/index?m%5BmaxCamber%5D=0&m%5Bsort%5D=5 which shows 2D infinite airfoils. The drag coefficient of airfoils continue to improve at higher speeds/bigger sizes (than bikes)
umop3pisdn 's example is only true at much higher speeds than 30mph
svenhöek
Both statements can be true, if the Cervelo graph is for an infinitely long tube with its axis at right angles to a homogeneous flow field and the "other source" is for a finite tube raked at bicycle tube angles in the practical bicycle flow field. The actual section of a circular down tube along the flow line is more like the ellipse represented by the second bar on Cervelo's graph.