Quote Originally Posted by Micropteris
If both casts are exactly the same from the same person with same techniques, same loops, same speed, same wind, same type of line (ie, floating WF), same leader, and same fly or bit of fuzz the heavier line will travel farther. It has more force and stored energy. As far as exact differences in footage I am not sure and don't really feel like trying to calculate it. Changing any of the other variables will screw with your results.
How about those (like me) who feel that when you start carrying a good amount of line outside of the rod tip it starts over-working the rod? Seems to me I can throw a lighter line further (on a normal rod... not a 5 weight that is really a 6 weight!) simply because the rod isn't overworked.