[Warning: stats-heavy post ahead. If you just want the payoff, skip to the last two paragraphs]

Going into the season, the narrative was about how 2016 could be one of Tennessee's best offenses ever. And it made sense. The Vols returned nine starters from a team that was in the top 30 in points per game last season. A dynamic quarterback returned. Both running backs returned. Four offensive linemen returned. There was continuity in the coaching staff. The Vols had all the ingredients for a dominant unit. We knew there were things that could be improved, most notably the Vols' ability to be explosive in the passing game. But the 2015 squad had proven that they were efficient enough to be very good even without much explosiveness. Big plays would just make a good offense into a dominant one. Or so was the narrative.

The problem is that it's wrong. At least by play-based measures, Tennessee's offense wasn't very efficient last year. Success rate, currently the best play-based measure of efficiency, is relatively simple as advanced stats go. It takes a concept we already know and love--third down conversion rate--and expands it to first and second down, setting goals for each of those downs that are about as difficult to achieve as a third down conversion. Those goals: 50% of the remaining yards on first down, 70% of the remaining yards on second down, and of course a conversion (100% of the remaining yards) on third or fourth down. The national average for success on each of these downs is between 40 and 45 percent. Overall, the average success rate is a hair under 42%.

So success rate measures a team's ability to slowly and steadily move the ball down the field. An efficient offense will do a better job of staying ahead of the chains and converting third downs than an inefficient offense. But Tennessee's offense, by this measure, wasn't especially efficient in 2015. Their success rate of 42.6% ranked just 57th in the country, sandwiched between 6-7 Nebraska and 3-9 Georgia Tech. And we know that the Vols weren't particularly explosive (81st in Bill Connelly's explosiveness rankings, for reference). But an offense, in order to be successful, should be good at either efficiency or explosiveness. So how do we explain Tennessee? What were the Vols good at?

It could be finishing drives. A mediocre offense that becomes very efficient in the red zone will look a lot better than it is. But it wasn't. The Vols were below average in finishing drives, scoring just 4.4 points per trip inside the 40 (ranking 69th nationally).

At this point, it's easy to see why Bill Connelly's numbers found the Tennessee offense wanting--the Vols failed to make the top 50 in any of the three statistical categories (explosiveness, efficiency, finishing) he weights most heavily. But the fact remains that the offense scored points. How did they do it? Brian Fremeau takes advanced stats from a different angle than Connelly, evaluating offenses on a drive-by-drive basis rather than a play-by-play basis, and his numbers were a lot higher on the Vols. So what does a glance at Fremeau's numbers tell us?

Well, it tells us that Tennessee was pretty good at "value drives"--drives that make it inside the opponent's 30--finishing 20th nationally in that statistic. They were also pretty good (26th nationally) at avoiding three-and-outs, going three-and-out just 23.2% of the time. But despite ranking in the top 20 in Fremeau's overall rankings, the Vols offense was just 48th nationally in scoring touchdowns on drives that recorded a first down. That's better than 57th or 81st, but it's still not excellent. So where did all these points come from?

There isn't just one answer, there are three. Between Connelly and Fremeau's rankings, there are three (full game, unadjusted for opponent quality) statistics in which the Vols offense ranked better than 20th: pace of play (16th), turnover rate (11th), and average starting field position (9th). The first explains why the Vols may have looked better at scoring points or amassing yardage than they actually were--they ran more plays than most other teams, so it should come as no surprise when they (and their opponents) score more than average.

But it's the second and third that tell where Tennessee's genuine successes have come from. The Vols took care of the football, and they had a really good special teams unit. The former actually includes a bit of good fortune--Tennessee's 20 fumbles were just about average nationally, but the Vols only lost 7, a sterling 65% recovery rate that may be difficult to duplicate. But the Vols did do a good job of recovering their own fumbles, and Josh Dobbs did a good job of avoiding interceptions, throwing just five on the season. Together, this allowed Tennessee to have more success on a drive-by-drive basis than they did on a play-by-play basis.

The offense's biggest weapon, on the other hand, had nothing to do with the offense. Cam Sutton, Alvin Kamara, and Evan Berry were the best return trio in the country, and together they helped the Tennessee offense to some of the best field position in the country (and make no mistake, this is because of special teams--the defense had one of the worst turnover rates in the country). This explains why Tennessee did such a good job of penetrating well into opponent territory--they were starting closer to the 40 than to the 20.

So why was Tennessee's offense so good in 2015? The fact is, it wasn't--at least not as good as the fan base thought it was. There were flashes of brilliance (see: the Georgia game and the first quarter of pretty much every game), but on the whole, the offense couldn't hang its hat on either efficiency or explosiveness. Its success came from a quarterback that took care of the ball, better-than-average fumble luck, and the good fortune to share a uniform with an exemplary special teams unit.

What does this mean for 2016? Well, nine starters and the coaches return, so there will likely be some continuity. So far, the Vols have been much less efficient than last year (thanks, offensive line), albeit in a tiny two-game sample. They recorded a few big plays against Virginia Tech, and so far the fumble luck and field position seem to be continuing into 2016. If the efficiency returns to last season's level and last week's big plays persist, a team can be successful with the sort of offense Tennessee can expect this season. Alabama won a title last year without being any more explosive and only moderately more efficient than the 2015 Vols. But if the defense and special teams can't carry the load alone, Tennessee might need a bigger step forward on offense than we may have thought.