All my rides have hills on them and so I pretty much always hit at least 65 km/hr (40 mph) unless its raining or in the spring and the roads have sand on them (lots of curves on the down hill). This is approaching fast, but fast for me is 80 km/hr. (50 mph).

There is a really good video on Utube of some guys in the tour doing 100 km/hr. Wish I had the link. It needs to be steep to go that fast, you (or at least I) can't spin that fast.