cavendish has said that he has been clocked at 76 km/h, pretty much about 47 mph.
http://www.youtube.com/watch?v=eO82Y...eature=channel
and if you watch the overhead shots, you can see the lead outs and then the jump at the end...the lead outs are going 40 mph and then you can see the jump in speed. 53x11 + spin fast!