I haven't done any research on it, but I have an out and back ride that a constant headwind often happens on.... how does that happen you say. Well the wind direction generally changes around mid day so if you go out and ride at the right time you'll get a headwind out, stop have some lunch and get a headwind home... I suppose if I changed to a route that went the other direction I could have a constant tailwind??? Never seems to work that way.