It would be inaccurate to try and derive a 40 yard dash time from a 100 meter time. In the 100 meter dash, you have 4 phases: Drive (0-15 meters), Accelerate (10-50), Top Speed (40-70), and Maintenance (60-100). The meters on the phases overlap because different runners run it differently. Thus trying to reduce someone's 100 m time proportionally to 40 yards would be off because they might reach top speed at 40 meters, and maintain it the rest of the race, which would mean more time spent at top speed, while the 40 yard dash is a measure of acceleration.
Speaking of 40s, they're trash, and this is why, using Michael Vick vs. Maurice Greene as an example. In a 40 yard dash you have certain factors like hand-timing and self starting. In the track world, a hand-time result has to add .24 seconds for human-error-adjustment. In self starting, you eliminate the reaction time to a gun-started event. Maurice Greene's reaction time at the '99 world championships 100 m, which he won in 9.80 seconds, was .139 seconds, rounded to .14 seconds. Add in these factors and, in a track setting with electronic timing, Michael Vick's 4.2 40 is worth a 4.58, not that impressive when compared to Maurice's 40 yard split in that race of 4.08. Going backwards, taking out reaction time and added the benefit of hand timing, Maurice's 40 yard turns into a 3.7 in a football combine setting.
The next time you hear an announcer say that football players are faster than olympians, unless they quote you a 3.7-3.8 time for the 40, then promptly dismiss the claim.
