Biggest improvements 

                                           Change in
    All Time:                               win %
  1 Indianapolis Colts     1998 -> 1999     0.625   (0.188 -> 0.813)
  2 New England Patriots   1975 -> 1976     0.572   (0.214 -> 0.786)
  3 Indianapolis Colts     1974 -> 1975     0.571   (0.143 -> 0.714)
  4 Pittsburgh Steelers    2003 -> 2004     0.563   (0.375 -> 0.938)
  4 St. Louis Rams         1998 -> 1999     0.563   (0.250 -> 0.813)
  6 Chicago Bears          2000 -> 2001     0.500   (0.313 -> 0.813)
  6 Indianapolis Colts     1991 -> 1992     0.500   (0.062 -> 0.562)
  6 New York Jets          1996 -> 1997     0.500   (0.062 -> 0.562)
  6 San Diego Chargers     2003 -> 2004     0.500   (0.250 -> 0.750)
 10 Cincinnati Bengals     1987 -> 1988     0.483   (0.267 -> 0.750)
 11 Tennessee Titans       1966 -> 1967     0.465   (0.214 -> 0.679)
 12 Miami Dolphins         1969 -> 1970     0.464   (0.250 -> 0.714)
 13 Oakland Raiders        1981 -> 1982     0.451   (0.438 -> 0.889)
 14 Baltimore Ravens       2005 -> 2006     0.438   (0.375 -> 0.813)
 14 Denver Broncos         1990 -> 1991     0.438   (0.312 -> 0.750)
 14 San Diego Chargers     1991 -> 1992     0.438   (0.250 -> 0.688)
 17 Atlanta Falcons        1997 -> 1998     0.437   (0.438 -> 0.875)
 17 Detroit Lions          1979 -> 1980     0.437   (0.125 -> 0.562)
 17 New Orleans Saints     1999 -> 2000     0.437   (0.188 -> 0.625)
 17 New Orleans Saints     2005 -> 2006     0.437   (0.188 -> 0.625)
 17 San Francisco 49ers    1980 -> 1981     0.437   (0.375 -> 0.812)
 22 New England Patriots   1981 -> 1982     0.431   (0.125 -> 0.556)
 23 Cleveland Browns       1975 -> 1976     0.429   (0.214 -> 0.643)
 23 Tennessee Titans       1973 -> 1974     0.429   (0.071 -> 0.500)
 25 Indianapolis Colts     1986 -> 1987     0.412   (0.188 -> 0.600)

    Last season:
    Cleveland Browns       2006 -> 2007     0.375   (0.250 -> 0.625)
    Green Bay Packers      2006 -> 2007     0.313   (0.500 -> 0.813)
    Tampa Bay Buccaneers   2006 -> 2007     0.313   (0.250 -> 0.563)
    Dallas Cowboys         2006 -> 2007     0.250   (0.563 -> 0.813)
    Detroit Lions          2006 -> 2007     0.250   (0.188 -> 0.438)
    New England Patriots   2006 -> 2007     0.250   (0.750 -> 1.000)
    Washington Redskins    2006 -> 2007     0.250   (0.313 -> 0.563)
    Jacksonville Jaguars   2006 -> 2007     0.188   (0.500 -> 0.688)
    Arizona Cardinals      2006 -> 2007     0.187   (0.313 -> 0.500)
    Houston Texans         2006 -> 2007     0.125   (0.375 -> 0.500)
    Minnesota Vikings      2006 -> 2007     0.125   (0.375 -> 0.500)
    New York Giants        2006 -> 2007     0.125   (0.500 -> 0.625)
    Oakland Raiders        2006 -> 2007     0.125   (0.125 -> 0.250)
    Pittsburgh Steelers    2006 -> 2007     0.125   (0.500 -> 0.625)
    Tennessee Titans       2006 -> 2007     0.125   (0.500 -> 0.625)
    Indianapolis Colts     2006 -> 2007     0.063   (0.750 -> 0.813)
    Seattle Seahawks       2006 -> 2007     0.063   (0.562 -> 0.625)