Thirteen games into the season, we can declare that the 2012 49ers are fielding one of best running games in recent history. The sample size is big enough now that the 49ers' 5.29 YPC can no longer be called a fluke, and while that average has dropped a fair amount from earlier in the year, they are still on track to finish as a top-10 unit post-merger. For context, that 5.29 YPC figure puts the 2012 49ers in the vicinity of the sophomore seasons of Hall of Famers Eric Dickerson and Barry Sanders, at least in terms of efficiency.

But personally, I've been having a hard time believing it. The 49ers rushing attack, while amazing, doesn't feel dominant in the way that Chris Johnson did in 2009 or the way Adrian Peterson ran in his rookie year. Yet according to YPA, the 49ers rushing attack is actually more efficient than both those teams. So what accounts for the difference?

Well according to the numbers, it may be that the 49ers' don't produce the long, back-breaking runs which usually characterize dominant rushing offenses. Using Pro-Football-Reference, I looked at every rush that gained more than 10 yards this year, to see where the 49ers rank in terms of explosiveness. Why 10 yards? In my mind, any yards gained that far downfield are the sole product of the running back -- 10 yards past the line of scrimmage, the offensive line has stopped blocking, and its the running backs' job to avoid second level defenders. The elite backs, the ones who go down in history, make their would-be tacklers miss.

First, the good news: Frank Gore leads the league in rushes that gain at least 10 yards, once you control for total rushing attempts. 19.7% of the times Gore has ran the ball in 2012, he's gained at least 10 yards. That's a full percentage point better that CJ Spiller, who's often declared as the most explosive back in the league, and even better than Adrian Peterson during his potentially record setting 2012. Kendall Hunter is not far behind at 16.4%, which is a testament to his growth as a running back and the 49ers' dominance along the offensive line. Harbaugh and Roman deserve a lot of credit as well -- they've put together a rushing attack that gets first down yardage almost 20% of the time.

And now for the bad news: The 49er running backs are below average once they get to the second level of the defense. Among qualifying running backs, Gore ranks 27th out of 40 in YPC on his runs of 10 or more yards -- although he passes the 10 yard mark more regularly than any other back in the league, he only averages 5 more yards before being tackled. That's well below Spiller, who's averaging 11.5 more yards after first down yardage, and Peterson, who's gaining a ridiculous 13.77 YPC after getting the initial ten yards. Hunter is even worse, averaging 4.36 YPC after the 10 yard mark. If you think that its just a quirk in the YPC data, both 49er backs are below average at turning 10+ yard runs into 20+ yard runs as well. And even though Hunter and Gore have combined for 50 runs of 10+ yards, they've only broken three for touchdowns. By comparison, Spiller has scored four long TDs on half as many carries.

My explanation: Gore and Hunter are getting tackled from behind with more regularity than any of the other elite backs. These guys are great at finding holes and attack the edge, but they aren't outrunning anyone in the open field. The 49ers are arguably the best rushing offense in 2012 at moving the chains, but just like in the passing game, they lack a level of explosiveness that characterizes the best offenses in the league. Is this even a problem? If it is, will the lightning fast LMJ solve it? Or will Gore have to take his game to another level in the playoffs?

I can't answer any of that, but I can show you my data. Here it is:

A note on methodology: all of my data comes from Pro-Football-Reference. I excluded all goal line rushing attempts (defined as within the opposing team's 10 yard line) and also eliminated any running back with fewer than 50 total carries or with fewer than 10 carries over 10 yards. The data is current through week 14, although its been updated to reflect the running backs involved in Thursday night's game.