dayum! 40 mph! That's almost impossible. What kind of bike do you have? I get 27 mph max on downhill on pavement, with road tires on my mtn bike. It's a Jamis Durango Sport SX, from 2004. About 5 mins from me is a lagoon/forest preserve, with a dirt/mud trail that leads to a large dirt hill that has a lot of jumps and downhill tracks down it. I have disc brakes, which were no match for this hill. There is a path to get to the top of the hill, and my friend, with an identical bike, but with the stock mtn tires, got stuck going up it. Me, with my hybrid road tires, powered up that hill right past him. It's all in the gearing.
