College football's post-Week 8 SP+ rankings

If you were a top college football team and you weren't either on a bye week or named Michigan, you probably got a little nervous at some point Saturday. Lots of highly ranked teams struggled. Most of them avoided an upset -- sorry, North Carolina -- but we saw a decent amount of change in the updated SP+ rankings.

Only top-ranked Michigan remains in the same spot it was in a week ago -- the Wolverines expanded their lead on the field following a ruthless blowout of rival Michigan State -- and there are otherwise lots of ups and downs.

Below are this week's SP+ rankings. What is SP+? In a single sentence, it's a tempo- and opponent-adjusted measure of college football efficiency. I created the system at Football Outsiders in 2008, and as my experience with both college football and its stats has grown, I have made quite a few tweaks to the system.

SP+ is indeed intended to be predictive and forward-facing. It is not a résumé ranking that gives credit for big wins or particularly brave scheduling -- no good predictive system is. It is simply a measure of the most sustainable and predictable aspects of football. If you're lucky or unimpressive in a win, your rating will probably fall. If you're strong and unlucky in a loss, it will probably rise.

Here are the full rankings: