Originally Posted by Street rider

But on a BMX bike, you are sitting in an upright position. All the air resistance hits you more than it hits the bike, and you more or less act like a sail, slowing you down. With a bike that is larger and actually designed for it, you will be in an aerodynamic position, lessening the wind resistance on your body. Theres a reason that you see people regularly commuting on road bikes or touring bikes, not very often on BMX bikes.