I would look at it this way. If you have 40 feet of line out, the end of the line has to travel about 80ft (plus the bend of the rod), from the time you stop the forward cast until it is straight out in front of you. If that takes one second it would be going 80ft/s or 54.5 mph.
I just realized you asked about the speed of the rod tip not the line. I'd have to think about that.