The 40-yard dash has become the way for scouts to gauge an NFL prospect's speed, and the marquee event at the NFL combine, which began this week.
But why is it 40 yards? Why not the 25, or the 50-yard dash? According to Michael MacCambridge's America's Game: The Epic Story of How Pro Football Captured a Nation, Cleveland Browns coach Paul Brown came up with the number in the mid-1940s, figuring the 40 was a "more meaningful measure of true football speed" than the 50- or even the 100-yard dash, because it was about the distance a player would cover on a punt:
The origin of timing football players for 40 yards comes from the average distance of a punt and the time it takes to reach that distance. Punts average around 40 yards in distance from the line of scrimmage, and the hangtime (time of flight) averages approximately 4.5 seconds. Therefore, if a coach knows that a player runs 40 yards in 4.5 seconds, he will be able to leave the line of scrimmage when a punt is kicked, and reach the point where the ball comes down just as it arrives.
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According to Sports Illustrated, Brown used the 40-yard dash as a tiebreaker to figure out who could cover a punt. Then Gil Brandt made it a uniform test with the Cowboys in the 1960s, determining that the 40 was the best distance for skill-position players. Other teams quickly followed suit, and the rest is history.