Level ground/no wind, about 32 MPH. Same on both my road bike and my crappy/heavy hybrid - top speed really shouldn't depend on the weight of the bike, but aerodynamics. On the road bike I can SUSTAIN 32 MPH longer, but really not that long.

With a tailwind I can hit 34 to 36. Downhill I've been up to about 40. I just run out of gears with my old bike. I haven't had the road bike long enough to have it on a significant downhill in good weather.