I can't beleive no one has linked to this yet:
"Track coaches go to Pro Timing Days, and they see scouts starting their stopwatches with their thumb, which has a slower reaction time than the index finger. They see them crowding the finish line and anticipating – guessing, basically – when someone will cross it. They see running surfaces that weren't professionally measured or leveled. They see no starter's gun, no automatic timing device, no wind gauge.
Grizzled track coaches love to say that the "clock doesn't lie." Well, it does in football.
Say someone clocks a hand-timed 4.35 in an NFL workout.
The accepted standard to convert a hand-timed event to its automatically timed equivalent is to round up to the nearest tenth of a second – in this case 4.4 – and add .24 seconds. Now you're at 4.64.
Most football 40s don't go on a starter's pistol but on an athlete's motion. The average reaction time among elite sprinters (from the gun to the moment they exert pressure on the starting block's electronic pads) is about .15 seconds; for a football player with little track experience it probably would be closer to .2. Add that in, and you have 4.84.
Now say it's a breezy day and you're running with a tailwind. Say it's 10 mph. Accepted track tables say that would provide a .07-second advantage over 40 yards. Add it in, and your 4.35 is suddenly a 4.91.
There's no shame in running a 4.9-second 40, of course. World-class sprinters get a bad start or get a cold day, and they go through 40 yards in the high 4s, too.
http://www.signonsandiego.com/uniontrib/20050418/news_1s18forty.html