"A player's speed in the 40 doesn't really tell you how fast he is in a game....
The key thing to remember is that functionality is the most important evaluation of a player's ability to perform. No matter how fast he runs, how high he jumps or how well he scores on a test, his value can only be related to how functional he is on the field. And that is often something that cannot be precisely measured anyplace except on the practice field or, in some cases, in a game situation....
But the problem with the 40 is the game of football requires functional speed, not pure track speed. So functional speed is related to playing the game and responses to another moving object. Jerry Rice's functional speed is probably the very best in the history of football. But if you timed him in the 40, he would be over 4.5."
-----Bill Walsh