I've seen that for the speeds that ameteur road cyclists ride at 40mm is the best tire size for speed in the real world, crazy, right? rough roads make for rider fatigue which offsets the aero penalty. But I think that also assumes that we have a wide enough rim for a 40mm tire, for ideal aerodynamics the rim should be 105% of the measured width of the tire as this avoids/reduces turbulence at the tire/rim junction.