48 mph downhill. but the real question is: how fast can you go on the flats? did 38mph with a 15-18 mph wind with me. Although when you think about it, anyone can hop on a bike and build up speed on a downhill. the thing that separates us from the rest is your max speed while you are going uphill, with a 15 mph headwind, on a crappy potholed road in a drizzle. now that is a topic.