Originally Posted by Eric F

As I was riding this past weekend, observing my front wheel spinning below me, it occurred to me that nothing on my bike was moving faster through the air than the tread of the tire at that moment. I also realized that I hadn't read/seen anything discussing the effects of tire tread on aerodynamic drag. I would think the difference between a 25mm road tire and a 2.25" MTB tire would be somewhat significant because of both width and tread. Looking at just high-end road tires, how much difference would there be between slight variations of shallow patterns in what is considered a "slick" tread (Conti GP5000 vs. Pirelli P-Zero, for example)? Is it completely inconsequnetial, or are there a few watts difference?