A little mathematics to convert and compare, Chris Johnson runs 4.24 for 40 yrds. 1 yrd = .9144 of a meter. So 40 yrds = 36.576 meters. so to convert his time to a 40 meter time, 4.24 divide by 36.576 multiply by 40 gives Johnson 4.637 for 40 meters. Not including reaction time.
Now Bolts 40 meter split in Berlin was 4.64 and that included a reaction time of .146. So give both guys an equal reaction time and add it to Johnsons 40 meter time puts Johnson at 4.78 V Bolt 4.64. A distance difference of 1.293 meters in Bolts favour at the finish line.
Now for the NFL campers I will also do the reverse for comparison since the 40yrds is your yard stick (pardon the pun). Bolt runs 40 meters with a reaction time properly timed in 4.64 secs. Convert to yards that is 43.745 yards. So divide the 4.64 by 43.745 and multiply by 40 to get the 40 yard time = 4.242 now to even it out with johnsons time subtract Bolts reaction time of .146 = 4.096 . So Bolt runs the NFL combine type 40 yrds .15 of a second quicker than Johnson. Allow Bolt the same type timing and start the is used in the NFL combine and he probably goes sub 4.0 for the 40 yrds.
WE have a winner….bearing in mind also how accurate or un-accurate is Johnsons 4.24 really . The clock was started on first movement, which all us tracksters know is way quicker than the generous reaction time I have given him in my calculations, so if both guys were to go head to head from the blocks from the gun, the gap over 40yrds or even 40 meters would probably be .2 to .3.
A final bit of Mathematical hypothesising…..if Johnson shaved off the dreadlocks and took the 1 kilo of gold out his mouth , or if Usain did the reverse and took on the trinkets and Baubles of a Rastafarian Roma Gypsy would it make a difference to the times? Happy New Year.