I take what Phil and Paul say with a ~5 mph grain of salt, but 60 mph is absolutely possible on a straighter section of a steep European road. Also, Phil and Paul ALWAYS say "reaching speeds IN EXCESS OF ______" when they don't really mean it. Why are they saying they're exceeding "45-50 mph"? If you're going to give a range, just say they're exceeding 50 mph. Saying they're exceeding 3 mph is an equally, or more true statement. Right?

Spinning a 53x11 gear at 120 rpm will put you at 46.4 mph. Pros can efficiently spin faster than that for short periods of time, but there's nothing about being a pro that alters the laws of gravity, wind resistance, and general physics on a straight, downhill section of road. Even guys like Cancellara, or heavy sprinters can't pedal their bike up to 60 mph, because they can't pedal efficiently at 200+ rpm, or perhaps it just makes more sense to get aero, use no energy, and still go 55 mph. Point being, physics does everything over 50 mph or so, and the rider doesn't control the conditions outside of aerodynamics (the "tuck", wheels, frame, etc.), mass, and wind resistance (getting behind a motorcycle or something).

The fastest I've been on a bike is 59.9 mph, down a ~9% grade, with a stout tailwind. I weigh more than most pros at 165-170, and my bike weighs about 10 pounds more than theirs does on a mountain stage.

For more info, check out the Garmin Connect page for the pro team, and look at the max speeds on the left. These are often glitchy, and guys forget to turn them off after bikes are loaded on vehicles, etc., but it'll give you a good idea of how fast they go. http://connect.garmin.com/teamGarmin