As a matter of curiosity, are there any studies on how fast large, weighty objects in a tornado's debris field are capable of moving relative to the actual wind speed? It takes one second for a 205 mph tornado wind to travel the length of a football field. Smaller, lighter debris borne on that wind are presumably traveling at roughly that same speed. But as the size and mass of debris increases, friction and gravity have more of an effect. It's safe to say that a roof or a pickup truck entrained in that wind isn't moving at the same speed as a piece of insulation, and even a 2 x 4 probably isn't traveling quite that fast. But what percentage of the actual wind speed are such objects capable of fulfilling? I realize the question is simplistic, given the variables involved, and answers are liable to range from "It depends" to quite complex. I'm mainly interested in what thought and research has gone into the matter and whether it's a factor in damage surveys.