Biggest improvements 

                                           Change in
    All Time:                               win %
  1 Indianapolis Colts     1998 -> 1999     0.625   (0.188 -> 0.813)
  1 Miami Dolphins         2007 -> 2008     0.625   (0.063 -> 0.688)
  3 New England Patriots   1975 -> 1976     0.572   (0.214 -> 0.786)
  4 Indianapolis Colts     1974 -> 1975     0.571   (0.143 -> 0.714)
  5 Pittsburgh Steelers    2003 -> 2004     0.563   (0.375 -> 0.938)
  5 St. Louis Rams         1998 -> 1999     0.563   (0.250 -> 0.813)
  7 Chicago Bears          2000 -> 2001     0.500   (0.313 -> 0.813)
  7 Indianapolis Colts     1991 -> 1992     0.500   (0.062 -> 0.562)
  7 New York Jets          1996 -> 1997     0.500   (0.062 -> 0.562)
  7 San Diego Chargers     2003 -> 2004     0.500   (0.250 -> 0.750)
 11 Cincinnati Bengals     1987 -> 1988     0.483   (0.267 -> 0.750)
 12 Tennessee Titans       1966 -> 1967     0.465   (0.214 -> 0.679)
 13 Miami Dolphins         1969 -> 1970     0.464   (0.250 -> 0.714)
 14 Oakland Raiders        1981 -> 1982     0.451   (0.438 -> 0.889)
 15 Atlanta Falcons        2007 -> 2008     0.438   (0.250 -> 0.688)
 15 Baltimore Ravens       2005 -> 2006     0.438   (0.375 -> 0.813)
 15 Denver Broncos         1990 -> 1991     0.438   (0.312 -> 0.750)
 15 San Diego Chargers     1991 -> 1992     0.438   (0.250 -> 0.688)
 19 Atlanta Falcons        1997 -> 1998     0.437   (0.438 -> 0.875)
 19 Detroit Lions          1979 -> 1980     0.437   (0.125 -> 0.562)
 19 New Orleans Saints     1999 -> 2000     0.437   (0.188 -> 0.625)
 19 New Orleans Saints     2005 -> 2006     0.437   (0.188 -> 0.625)
 19 San Francisco 49ers    1980 -> 1981     0.437   (0.375 -> 0.812)
 19 San Francisco 49ers    2010 -> 2011     0.437   (0.375 -> 0.812)
 19 Tampa Bay Buccaneers   2009 -> 2010     0.437   (0.188 -> 0.625)

    Last season:
    San Francisco 49ers    2010 -> 2011     0.437   (0.375 -> 0.812)
    Green Bay Packers      2010 -> 2011     0.313   (0.625 -> 0.938)
    Cincinnati Bengals     2010 -> 2011     0.312   (0.250 -> 0.562)
    Carolina Panthers      2010 -> 2011     0.250   (0.125 -> 0.375)
    Denver Broncos         2010 -> 2011     0.250   (0.250 -> 0.500)
    Detroit Lions          2010 -> 2011     0.250   (0.375 -> 0.625)
    Houston Texans         2010 -> 2011     0.250   (0.375 -> 0.625)
    Arizona Cardinals      2010 -> 2011     0.188   (0.312 -> 0.500)
    Tennessee Titans       2010 -> 2011     0.187   (0.375 -> 0.562)
    Buffalo Bills          2010 -> 2011     0.125   (0.250 -> 0.375)
    Dallas Cowboys         2010 -> 2011     0.125   (0.375 -> 0.500)
    New Orleans Saints     2010 -> 2011     0.124   (0.688 -> 0.812)