According to IAAF Analysis Split Times, Usain Bolt ran his first 40m in 4.65 seconds. Subtracting his reaction time which was .165 seconds, he ran the 40m in 4.485 seconds. 40 yards is a totally different measurement than 40 meters. 40 yards is equal to approximately 36.576m

By simple ratio:

(40m/4.485)=(36.576m/X)

X=( 36.576 / (40/4.485) )

Usain Bolt's 40 yard dash would be: A blazing 4.100 seconds! Considering the human error in hand-timed timing used in most combine settings, this time could easily be read as 3.85 seconds at the NFL Combine!

DIFFERING VIEWPOINTS:

There are several errors in this presentation.

1. NFL official combine times are all electronic, there is no longer "hand-timing" error. Bolt's time above is for 40 meters which is longer than 40 yards, it is equivalent to 43.745 yards. Therefore the time indicated above is faster than his time would have been at 40 yards as he would still be accelerating. His time would have had to have been calculated at 36.576 for an accurate conversion. Given the misrepresentations above, it seems likely Bolt would be clocked in the range of 4.2-4.3 at the NFL combine. There are likely several Olympic caliber 100 meter sprinters that would best Bolt easily, as he is not known for his strong starts, with his long stride he powers past faster starters around and after the 50 meter mark. The fastest official time at the NFL combine is 4.24 for Chris Johnson.

1. NFL combine times are electronically stopped, but started by hand, in response to the runner starting to move forwards. In Olympic races, the runner reacts to a starting gun, meaning about 0.2 seconds extra of reaction time. In Bolt's case, 0.165 seconds. So it is correct to subtract this time. It's also worth noting track sprinting also uses starting blocks, which increase the very first bit of acceleration relative to what could be achieved without them, and thus reduce total time.
2. Bolt's time in the 40m is clearly greater than it would be for 40 yards, since it takes more time to cover a larger distance. It is true that since Bolt would still be accelerating for the 3.75 or so yards between 40 yards and 40 meters, simply multiplying his 40m time by 36.576/40 will slightly underestimate the time bolt covered the first 40 yards. But the change in speed over a distance of 4 yards around the 40m mark of the race translates into a very small change in time.Thus, the original answer is more correct than the revised one, though the effect of starting blocks and the minute adjustment due to the acceleration between 40 and 43.75 yards should properly be taken into account.

I did research comparing the 40 meter times and 100 meter times of extremely fast football players, such as Trindon Holliday, Jeff Demps, Bob Hayes, Deion Sanders, and Herschel Walker. For every 40 yard split of each athlete's 100 meter time, the split was, on average, about .5 seconds faster than the athletes' actual 40 times. I found a common ratio of 40 time to 40 yard splits in the 100: the actual 40 was about 1.142 times longer than the 40 yard split time. record 100 meter time, and using the ratio, I calculated Usain Bolt's probable 40 time to be 4.003 seconds. I know it sounds ridiculous, but the math is correct. It is not an exact science, but it is a very good estimate.

However, don't forget that NFL players don't use "blocks" when doing the 40 as they do in track. They are more prone to "rising" faster out of their stance since they have to push off of the ground instead of the "blocks." Blocks will improve your time.