Years ago, I broke 50 on one of my road racers, going down a particularly nasty route on our resident mountain. Now that my racing/heavy training days are over, I'm running some pretty wide, comfy and slow Continentals, on some relatively heavy SwissDT rims, w/ 105 hubs.
I hit 40mph the other day, on a winding route down the mountain I don't usually take, but that was unusual (and right at the edge of scary, given the turns), but I typically have to pedal like I really mean it to get over 35, with these wheels and tires.