Essentially, anyone below the trendline is succeeding at the 40 given their weight, and anyone above it isn't. And the larger the man, the slower the man.
Now, weight isn't the only variable in determining how fast a runner is, but it's a variable. So given this trendline and the fact that we now have weights of this year's running back class, we can start to see how fast these backs will be expected to run their 40-yard dash.
Here's a list of running back combine participants, their weight, and the average 40-yard dash time a running back has run since the 2005 combine given that specific weight.
With players like Leonard Fournette and Dalvin Cook, this isn't saying that I'm expecting them to run 40 times at the specific speed listed. Fournette should easily exceed the 4.617 time shown above, and that's part of the reason he's arguably the top running back in this loaded class.
What this does show is the average. There haven't been a ton of hefty, 240-pound backs at the combine over the last 10-plus years, but amongst them, the average has been well above a 4.6-second 40.
This is kind of just a checklist. It's a gut check to make sure these players are on the right path.