In 2008, the Steelers rank #1 in points allowed (13.9/game) and #1 in yards allowed (237/game). They also rank #1 in pass yards per attempt allowed and #1 in rush yards per attempt allowed.*
It is not just conventional numbers that show the Steelers as the best in the league. The Steelers rank #1 in Football Outsiders Defense DVOA, #1 in in Brian Burke's defensive efficiency rankings, #1 in Cold, Hard Football Facts Hog Index, and #2 in CHFF Defensive Passer Rating.
This is a defense that is basically the best in the league at everything.
How does this Steeler defense compare to earlier Super Bowl Steeler teams?
1974: 13.5 points per game (#2), 219.6 yards per game (#1)
1975: 11.6 points per game (#2), 295 yards per game (#1)
1978: 12.2 points per game (#2), 260.5 yards per game (#3)
1979: 16.4 points per game (#5), 266.9 yards per game (#2)
1995: 20.4 points per game (#9), 285.1 yards per game (#3)
2005: 16.1 points per game (#3), 284 yards per game (#4)
But the 2008 team is not the first Steeler team to rank #1 in both points allowed and yards allowed: the 1976 Steelers and 2004 Steelers both achieved that feat.
Since the merger, four Super Bowl champions have fielded a defense that ranked #1 in both points allowed and yards allowed:**
Perhaps the 2008 Steelers will be joining these teams as the fifth.
*NFL.com ranks the Vikings #1 and Steelers #2 with 3.3 ypa, but by the numbers they provide the Vikings allowed 3.31 per attempt, the Steelers 3.29.
**Unnecessary cheap shot to remind you this is, after all, a Viking blog: Brett Favre has a lot in common with Brad Johnson, Jim McMahon, and Bob Griese--give him an historically great defense, and he can win a Super Bowl!