For me, the most impressive and potentially meaningful thing about the Royals start is not their perfect record but their ridiculous run differential (RD). They have obliterated the White Sox, Angels, and Twins to the combined tune of 52 runs scored and 18 allowed. Since 1900, their +34 RD is tied for the fourth best after the first seven games of a season, and it represents the best RD a team has had after seven games since 1962. Only 17 squads since 1900 have had a +30 or better RD at this point in the season. (Unfortunately one of those teams is the 2015 Tigers.)

Here’s a look at those 17 teams, along with their final record and postseason performance:

Team Year 1st 7 RS RA RD Final W-L Postseason
St. Louis 1962 7-0 70 28 42 84-78 -
NY Giants 1905 6-1 57 17 40 105-48 WS Champ
Brooklyn 1940 7-0 46 11 35 88-65 -
San Fran 1962 6-1 59 25 34 103-62 NL Pennant
Kansas City 2015 7-0 52 18 34 ? ?
Chi Cubs 1934 7-0 44 11 33 86-65 -
Yankees 2003 6-1 61 28 33 101-61 AL Pennant
Cleveland 1999 6-1 63 30 33 97-65 Division Champ
San Fran 2002 6-1 41 8 33 95-66 NL Pennant
Yankees 1927 6-0-1 55 23 32 110-44 WS Champ
Yankees 1999 6-1 48 17 31 98-64 WS Champ
Mets 2007 5-2 47 16 31 88-74 -
Cleveland 1920 6-1 59 29 30 98-56 WS Champ
Detroit 2015 6-1 51 21 30 ? ?
Seattle 1995 6-1 48 18 30 79-66 Division Champ
Phillies 1915 7-0 39 9 30 90-62 NL Pennant
San Diego 1996 5-2 57 27 30 91-71 Division Champ

Holy Moses. It’s almost scary how good most of those teams went on to be. Ignoring the 2015 Royals and Tigers, the other teams have a combined final winning percentage of .599, which is a 97 win season. Only four of the 15 teams missed out on postseason play.

Another way to look at the potential significance of KC’s RD is to find recent teams that have had a +34 or better RD during any seven game stretch in a season (not just the first seven of the year). (Rob Neyer has an interesting question about whether or not all streaks are created equal.) Over the last five years, 2010-14, there have been just 27 teams to put up such a good RD during any seven game stretch, or an average of five or six teams a year. Those 27 teams ended the year with an average of 88 wins, and 14 of them made the postseason. Holy Moses.

These numbers seem pretty exciting to me, but there are also a few teams that serve as cautionary tales. You may have noticed in the table above that the team with the best ever RD start to a season, the ’62 Cardinals, ended up with a just OK 84 wins and out of the playoffs. And over the last five seasons, three truly terrible teams have fluked into dominant seven game stretches at some point (2010 Twins, 2014 Red Sox, and 2014 Rockies). So of course this great start guarantees nothing. But more often than not, teams that have a stretch of seven games this good are for real.

Shout out to the magical Baseball Reference Play Index.