Biggest improvements from one season to the
next (based on regular season win percentage):
                                          Change in
       Franchise                            win %
  1 Arizona Diamondbacks   1998 -> 1999      21.6   (40.1  ->  61.7)
  2 Boston Red Sox         1945 -> 1946      21.4   (46.1  ->  67.5)
  3 Atlanta Braves         1935 -> 1936      21.3   (24.8  ->  46.1)
  4 Philadelphia Phillies  1904 -> 1905      20.4   (34.2  ->  54.6)
  5 Baltimore Orioles      1988 -> 1989      20.2   (33.5  ->  53.7)
  6 Philadelphia Phillies  1961 -> 1962      19.8   (30.5  ->  50.3)
  7 San Francisco Giants   1992 -> 1993      19.2   (44.4  ->  63.6)
  7 Tampa Bay Rays         2007 -> 2008      19.2   (40.7  ->  59.9)
  9 Pittsburgh Pirates     1917 -> 1918      18.9   (33.1  ->  52.0)
  9 St. Louis Cardinals    1913 -> 1914      18.9   (34.0  ->  52.9)
 11 Oakland Athletics      1946 -> 1947      18.8   (31.8  ->  50.6)
 12 Cincinnati Reds        1937 -> 1938      18.3   (36.4  ->  54.7)
 12 Minnesota Twins        1911 -> 1912      18.3   (41.6  ->  59.9)
 14 Boston Red Sox         1911 -> 1912      18.1   (51.0  ->  69.1)
 14 Philadelphia Phillies  1928 -> 1929      18.1   (28.3  ->  46.4)
 16 Arizona Diamondbacks   2010 -> 2011      17.9   (40.1  ->  58.0)
 16 Atlanta Braves         1952 -> 1953      17.9   (41.8  ->  59.7)
 16 Atlanta Braves         1990 -> 1991      17.9   (40.1  ->  58.0)
 16 Detroit Tigers         2003 -> 2004      17.9   (26.5  ->  44.4)
 16 Oakland Athletics      1979 -> 1980      17.9   (33.3  ->  51.2)
 21 Oakland Athletics      1908 -> 1909      17.7   (44.4  ->  62.1)
 22 Chicago Cubs           1966 -> 1967      17.6   (36.4  ->  54.0)
 23 San Francisco Giants   1953 -> 1954      17.5   (45.5  ->  63.0)
 24 St. Louis Cardinals    1903 -> 1904      17.3   (31.4  ->  48.7)
 24 Texas Rangers          1973 -> 1974      17.3   (35.2  ->  52.5)

    Last season:
    Arizona Diamondbacks   2010 -> 2011      17.9   (40.1  ->  58.0)
    Milwaukee Brewers      2010 -> 2011      11.8   (47.5  ->  59.3)
    Pittsburgh Pirates     2010 -> 2011       9.2   (35.2  ->  44.4)
    Detroit Tigers         2010 -> 2011       8.6   (50.0  ->  58.6)
    Washington Nationals   2010 -> 2011       7.1   (42.6  ->  49.7)
    Cleveland Indians      2010 -> 2011       6.8   (42.6  ->  49.4)
    Los Angeles Angels     2010 -> 2011       3.7   (49.4  ->  53.1)
    Seattle Mariners       2010 -> 2011       3.7   (37.7  ->  41.4)
    Texas Rangers          2010 -> 2011       3.7   (55.6  ->  59.3)
    Philadelphia Phillies  2010 -> 2011       3.1   (59.9  ->  63.0)
    St. Louis Cardinals    2010 -> 2011       2.5   (53.1  ->  55.6)
    Kansas City Royals     2010 -> 2011       2.4   (41.4  ->  43.8)
    Baltimore Orioles      2010 -> 2011       1.9   (40.7  ->  42.6)
    Los Angeles Dodgers    2010 -> 2011       1.5   (49.4  ->  50.9)
    New York Yankees       2010 -> 2011       1.3   (58.6  ->  59.9)
    Boston Red Sox         2010 -> 2011       0.7   (54.9  ->  55.6)