Some say this comparison is an illusion. I don't think so at all.
Training on how to run the 40, on a composite track, in perfect conditions, and then wearing clothing that doesn't impede movement is one thing.
Running in cleats, a helmet, with pads, etc., on grass or even field turf, is another.
Some guys just seem to run as fast in a 40 as they do padded up. Others (I'd guess most) players are slower.
Kupp and Jefferson are 2 that run fast on grass with pads on. I'll take that every time since the game isn't played in shorts and track shoes.
And BTW: this is where the Combine stuff, for me, takes back seat to what happens on a field during a game. Forever we see super fast guys at the combine simply not ever become special, or even not that good, in the NFL. What matters is good enough speed, good quickness, good hands, route running, smarts, and toughness. And that's what the Rams want. Woods, Kupp, and Jefferson exemplify those attributes.
