There are so many modalities of 40 timing that the phrase "40 time" isn't, itself, very meaningful.
1) single stopwatch, entirely hand-timed, stopwatch begins on player's first movement
2) single stopwatch, entirely hand-timed, stopwatch begins on coach shouting "go!"
3) two stopwatches, entirely hand-timed, stopwatches begin on player's first movement
4) two stopwatches, entirely hand-times, stopwatches begin on coach shouting "go!"
5) touch-pad starts clock running, single stopwatch ends clock running
6) touch-pad starts clock running, multiple stopwatches end clock running
7) single stopwatch starts clock running on player's first movement, laser ends clock running
8) single stopwatch starts clock running on coach shouting "go!", laser ends clock running
9) touch-pad starts clock running, laser ends clock running (fully automatic)
Add in, for some of these: on grass/on track. Also: sneakers/cleats/track spikes. Also: indoor/outdoor
All these methods will yield very different times - adjusting for runner's reaction time (to "go!" call), timer's reaction time (to players first movement AND to when player crosses the line), speed of surface, footwear, etc
With track and field, there's a standardized way of timing for every serious meet - fully automatic timing. Football, in general, eschews fully automatic timing because it results in the "slowest" (most accurate) time. For this reason, it is very very difficult to compare 40 times - there's simply no way of knowing how it was timed and whether anyone is being honest about the results.
Comparing the slowest and fastest ways of timing: let's say we have one coach with a stopwatch, starting the clock when he sees the kid move and ending it when he thinks he sees the kid pass the line ...at 4.3.
Fully automatic timing would add in maybe .24 for the coach reacting to the movement, up to 0.1 to the coach anticipating the finish...now it's 4.64.
I have never trusted a 40 time.
