Well for starters there is a difference in the kind of top end speed that world class 100/200 m guys get to and the necessary 40 yd times of NFL players. After that there are just a tremendous amount of variables involved in how one applies their speed in football or any contact sport. The university of texas had a RB by the name of charles who was a top notch sprinter. He was not so great between the tackles but when he got in the open field it was good night irene.