Why so? sitting on the bike, arms out, a human is basically a misshapen parachute when riding. Put a windshield on it, now the wind is directed around, rather than being caught against the body. From an engineering standpoint, particularly when compared to cars, motorcycles are aerodynamic disasters. Even a sportbike, when compared to a sports car, has a considerably worse drag coefficient.
Anyhow, that has remained a constant, hanging steady at 52.5mpg for my last 3 fill ups.
