Notice that:
1. There are two closeups of his crankset while he is drafting the truck. He is in the big ring in the first (40mph) closeup, but in the small ring in the second (50mph) closeup.
2. In some of the frontal views (where you can see the actor's face), he is pedalling less than half as fast as he would need to go 60mph.
Still, I think this is one of the best scenes in the movie, and this is the best US-made bicycling movie ever.