Well, the "average" man cannot ride a bicycle 170 km in one day unless perhaps there is a gun to his head, so I assumed that the OP meant the "average man who is capable of riding 170 km in one day". IMO, that cyclist should be able to average a bit more than 25 km per hour, at least on a road bike. As I mentioned, I have no idea what a good average for a mountain bike is because a mountain bike is a poor tool for that job and I can't really imagine anyone wanting to do long road miles on one. But then I see people driving around on the freeway in Hummers so I apparently don't know much.