This thread made me curious about my own speed - since I got the Garmin edge four months ago, I don't have any other recorded speed before then. The Garmin Connect summary shows a max of 48.3mph, but after a closer examination using the Player function, there is no such data point! Ignoring that, my top speed is 46. But what this demonstrates is that you can't always trust your gizmo's recorded speed since apparently it can be much lower than recorded. I am not sure why this is the case because I thought that Garmin records the speed continuously. I guess it's possible for it to freeze sometimes or maybe the magnet sensor doesn't always work reliably, especially at faster wheel rotations. So if you want to know your true max speed, I think you need to look at the data more closely. The Player function in Garmin Connect has the feature to advance to the next data point. And by the way, I have set up my Garmin Edge 500 to record the GPS coordinates for every second, so it's the highest resolution that can be achieved on that device anyway.

As far as going over 40mph, I am able to do it on almost every ride. Even if the descent isn't steep enough, my 52x11 compact allows me to pedal hard enough to get there. It's like an opportunity for interval training that's actually fun. Other than what was said about cresting and aerodynamic position, it's also important how well you can corner with minimal or no braking. There are great youtube videos demonstrating the correct way to corner at speed.

Someone seemed to imply that aero wheels don't matter on the descent, but I disagree. The entire functionality of those wheels comes into play at speeds over 25mph, so a descent at over 40mph is the perfect place where an aero wheel can provide the most advantage.