The Rivalry

There’s a long, long history between the Hated Yankees and the Red Sox, and as any baseball fan can tell you, over that time the Yankees have tended to outplay the Sox down the stretch run, even when the two teams appear to be evenly matched.
OK, we know that. But by how much? Let’s look at the numbers. I went through every season since the Yankees’ 1903 arrival in New York in which both teams were in some sense still in the pennant race – i.e., both were within 10 games of first, or one was and the other was just a few games behind them – on July 31, and then compared their records from August 1 through the end of the season. Overall, that amounted to 38 seasons.
The net result? In the 38 seasons, the Sox were 2130-1676 (.560) through July 31, and the Yanks were 2190-1615 (.576). But from August 1 on, the Sox teams slowed to 1255-1070 (.540), while the Yankees heated up to 1360-968 (.584). Overall, the Yankees gained ground on the Sox, in absolute terms, 23 times, while the Sox gained ground 15 times (oddly, not once did they share the same post-August 1 record; the closest seasons were in 1904, 1948 and 2000, when the two teams were separated by a half game down the stretch). The biggest gains for the Yankees were 1952 (+14.5 games), 1985 (+13.5 games), 1937 (+12.5 games), and 1955 and 2001 (+10 games). The biggest gains for the Sox were 1973 and 1991 (+12.5 games), 1916 (+9 games), 1972 (+6.5 games), and 1949 (+6 games). The longest number of consecutive seasons in the study when the Yankees gained ground: 8, from 1934 to 1945. Longest for the Sox: 4 from 1986-91.
Of course, the Yankees, on average, started ahead. 1937 is probably the season in the study closest to the edge: the Yanks had a 9 game lead on the Sox, and were never really in any danger of not winning the pennant. Even in relative terms, though, the effect held up: the Yankees did better than the Sox relative to their winning percentage through 7/31 in 25 of 38 seasons.
Let’s break the numbers out by groups of seasons:

Seasons in study: 1904, 1910, 1916, 1934, 1935, 1937, 1938, 1939, 1944, 1945, 1948, 1949, 1950, 1951, 1952, 1955, 1970, 1972, 1973, 1974, 1977, 1978, 1979, 1982, 1983, 1985, 1986, 1988, 1989, 1991, 1993, 1995, 1999, 2000, 2001, 2002, 2003, and 2004.