When it comes to descending, professional cyclists can reach speeds of up to 60mph. However, the average speed that they descend at is usually around 30-40mph. There are a few factors that come into play when it comes to how fast a cyclist can go downhill. The first is the weight of the rider and their bike. The heavier the rider, the more momentum they will have and the faster they will be able to go. The second factor is the surface of the road. Smooth pavement will allow for faster speeds than rougher roads. Finally, wind conditions can also affect speed. If there is a tailwind, riders will be able to go even faster than usual.