Georgia assumed the No. 1 spot in the SP+ rankings four weeks into the 2021 season, briefly ceded it to a torrid Ohio State - more specifically, a torrid Ohio State offense - midway through the year, then pulled away from the field late in the year. The Dawgs couldn't reach the finish line 15-0, but they were still the class of college football, and they finish the year as both the national champions and the top-ranked team in SP+.
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.
It is, as always, important to note that SP+ is 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. It is a look at how teams should likely perform moving forward.