Perine is a great back, but he did most of his damage against garbage teams. And, while all backs do this, his was disproportionately split. If Oklahoma played real teams, with real defenses, his numbers would look much more human. And this was, again, with his NFL caliber O-line last year. Now he is running behind a very weak, and inexperienced, line.
Here are the rushing defense rankings of teams that Perine ran for at least 100 yards and 5 YPC against: West Virginia - 65th; Iowa State - 123rd; Texas Tech - 124th ; Kansas - 106th; Oklahoma State - 59th ; Clemson - 5th.
Only one school was in the top 50, two in the top 60. And the average was 80.3.
Here's some extra info. Here are his rushing averages against top 70 defenses: Perine: 104 carries, 460 yards, 4.42 YPC. To put it in perspective (and to show that not all "elite" backs thrive solely by beating up on bad teams, here are Chubb's numbers against top 70 run defenses: 181 carries, 1142 yards, 6.31 YPC.
Edit: Against top 70 defenses, Hurd ran 115 times for 555 yards. That's more carries, more yards, and a higher YPC (4.82). And, that is WHILE running behind a MUCH WORSE O-line. So, against good defenses, Hurd was better last year than Perine. And, of the "top 70" rushing defenses that each played, here is the average rankings: Perine - 31.8; Hurd - 29.5. So, the "good" teams that Hurd played had, on average, a higher ranked run defense than the "good" teams that Perine played. Point Hurd.
And, again, Hurd had a porous o-line that is now improved, and Perine had a fantastic O-line that has now disappeared. Moreover, Hurd added over 20 lbs of muscle, while not losing any speed. Point Thundering Hurd.