The Auburn Tigers are famously unpredictable, but if they’ve had one historical constant, it’s been a physical running game, with a lineage including Bo Jackson, Cam Newton, and so many other great runners.

In three of Gus Malzahn’s first five years as head coach, the Tigers finished between fourth and 17th nationally in yards per carry, plus two top-11 finishes in his three years as OC.

His Tigers have never been as bad as they’re pacing to be through six weeks of 2018: 82nd out of 130 FBS teams at a 4.18-yard average. And they haven’t even gotten all the way into the hardest part of the SEC schedule yet.

Running well is supposed to be Malzahn’s thing, and it was Auburn’s thing even before he came along. He’s been one of the sport’s savviest coaches at the ground game, drawing on both newer spread principles and the old single wing. So, let’s take a stab at why Auburn’s suddenly so mediocre at it.

We should start with the basics. Auburn’s running game is failing on both of the two main fronts: efficiency and explosiveness.

Auburn is 100th out of 130 FBS teams in Rushing Marginal Efficiency. Most simply put, the Tigers are doing a bad job of staying on schedule with solid gainers.

Auburn is 76th in Rushing Marginal Explosiveness. Even when the Tigers’ runs are successful, they’re not home runs.

Here are more detailed explanations or both of those stats. Marginal Efficiency measures how often a team gets successful plays — 50 percent of the necessary yardage on first down, 70 percent on second, and 100 percent on third or fourth — relative to other teams in similar downs and distances. Marginal Explosiveness measures how much additional yardage a team gets on the plays that are successful.

Auburn’s offensive line hasn’t been great. But the line is hurting efficiency more than it’s hurting explosiveness.

This threshold varies, but a good rule of thumb is that the offensive line is responsible for the first 5 yards beyond the line on any run. That’s about how much space a line can give a ball-carrier by blowing open holes and pushing defenders backward.

Auburn’s Opportunity Rate, the percentage of runs to reach 5 yards, is 48.3 percent — 50th in the country. That’s a mediocre number, but it’s not new for Auburn. The three years before this, the Tigers were 43rd, 40th, and 64th in Opp Rate. The line isn’t giving Auburn’s backs a lot of chances to rip off big runs, but that’s been true for years.

2018’s line has taken a big step backward in even letting the Tigers’ ball-carriers get started. Auburn’s Stuff Rate — or how frequently the Tigers avoid being tackled at or behind the line — is 18 percent, which ranks 52nd. The three years before 2018, they finished ninth, 17th, and ninth again.

For years, Auburn’s rushing game was reliable for at least getting something almost every time. Not so anymore.

The Tigers have had a hard time adjusting with a lot of new linemen.

The first thing all of this evidences is that Auburn’s run game is not add-water-instant road grading. The Tigers lost All-American right guard Braden Smith, as well as perennial starters Austin Golson (left tackle), Casey Dunn (center), and Darius James (right tackle).

They added UMass transfer Jack Driscoll at right tackle and elevated other inexperienced linemen up. It’s just not going as planned.

Auburn’s biggest problems reside in the interior, where it really misses Smith, Dunn, and former running back Kerryon Johnson — all three of whom are on NFL rosters. When Auburn has success running these days, it’s more likely to come outside the tackles.

Against Washington, you can see both guards get stoned, and there’s nowhere for fullback Chandler Cox to go.

Let’s take a third-and-short play from the LSU game. This is a play Auburn has to have, where pretty much everyone in Jordan-Hare Stadium knows it’s going to be a run play. Great offensive lines plow the road anyway.

But interior pressure can ruin an otherwise promising play. On third down here, Auburn tries to mash. But instead, LSU does the mashing. Right guard Mike Horton gets blown off the ball, disintegrating any hopes of the double team it looked like he and center Kaleb Kim were about to execute. Kim can’t get to the second level effectively, the pressure comes through, and it’s a tackle for loss.

They fail to get the job done on fourth down as well, and come away without any points in a game they lost by one point.

Later in the game, on Kim’s other side, he and left guard Marquel Harrell get stalemated by LSU’s Rashard Lawrence, who was a force against the run all day.

And this is a basic power run, with a pulling lineman:

Kim, the center, is asked to block back on Lawrence, because the left guard is pulling with a bit of help from the tight end. (There’s no true left tackle here; he’s on the other side, as Auburn uses unbalanced lines quite a bit.) Kim is capable of making those blocks, and he does it here without a problem.

But on the very next play, Auburn runs the same action with a stunningly different result:

Lawrence’s get-off is exceptional, but this block is not impossible, given Kim just did it. Credit the running back for getting this back to the line of scrimmage.

A lack of immediate force from the line seems to be a real issue. In addition to sporting a worse Stuff Rate than usual, Auburn’s really struggling on short yardage. The Tigers are 112th in third-and-short Success Rate (56 percent) and 85th in goal line Success Rate (60 percent). In situations where an elite recruiting team should just be bigger and stronger than the opposition, the Tigers are getting turned back way too often.

Malzahn’s Tigers have never been about ripping off big runs, and they’re still not.

The last three years, the Tigers ranked between the 60s and the 120s in Bill Connelly’s Rushing IsoPPP, a stat that measures ground game explosiveness.

That fits with how most people would perceive Auburn’s feature backs under Malzahn. Tre Mason, Cameron Artis-Payne, Peyton Barber, Kamryn Pettway, and Johnson were all stout bulldozers whose M.O. was keep the offense on schedule. The Tigers don’t rely on shifty backs trying to go the distance.

Auburn’s top two backs in 2018, JaTarvious Whitlow and Kam Martin, aren’t home run hitters. On Whitlow’s 5-yard carries, he averages 4.4 additional yards before he’s tackled. We call those highlight yards. On Martin’s, he averages 2.8 highlight yards. The national average is close to 5.

Here’s a 3-yard run in the red zone against Washington:

Not a bad gain, and there’s a nice hole here for Martin to get something, but what if the LG, Harrell, could have gotten a bit of a push and held this block on No. 96 in purple? Maybe Martin squeezes through and bangs out a first down.

The Tigers have one ball-carrier who’s turned up a lot of big plays: 5’7, 173-pound freshman Shaun Shivers, who’s averaged 5.6 highlight yards. Shivers gets fewer than half the carries either of the other guys gets, but he can get himself into space more quickly:

The Tigers’ down-to-down efficiency has just gone way downhill.

Auburn being 100th in a key rushing stat is jarring. The Tigers not being able to reliably get 4 yards on the ground is an unusual sight.

And no, these struggles can’t really be chalked up to Auburn facing eight or even seven defenders in the box a majority of the time. Since Auburn can very much still beat you on the perimeter, defenses do still honor the Tigers in that way.

All of this wouldn’t be the hugest deal for some teams, but Auburn makes its bones running the ball consistently and effectively. The schedule is only getting harder. If Auburn can’t run the ball against Alabama, Georgia, or Texas A&M, it’ll make for some long days at the office.