The defending World Series champs don’t have the best record in baseball. The Astros, at 35-20, are tied for third. By the time you read this, they may well even be out of first place in the AL West, with the red-hot Mariners nipping at their heels and the week’s schedule infinitely more favorable for Seattle than Houston. Their .636 win percentage is good, certainly, but not dominant. But one area where they are dominant? The team’s run differential, at +126 (at the time this piece was published), which has them on pace to set a record that’s been in place since before the U.S. entered World War II.

Run differential is a simple stat: It’s the difference in runs scored versus runs allowed throughout the season. It’s an imperfect metric, but it allows for a broad analysis of which baseball teams are the most powerful. After all, winning or losing a one-run game usually comes down to luck, but tracking who has got the most active bats relative to their defense offers a perspective of which team is actually playing the best baseball over the course of the season. And in that regard, the Astros—who have forty more runs than the next-best team—aren’t just the best in baseball right now. They’re on pace to be one of the best teams in baseball history.

The best run differential in MLB history is held by the 1939 Yankees. That record hasn’t been broken in nearly eighty years—back when Joe DiMaggio, Bill Dickey, and Red Ruffing were leading the Yanks to claim the title of the best team of all time, The Wizard of Oz was premiering in movie theaters and World War II was just beginning. Since then, several teams have boasted good run differentials—the 1998 Yankees had +309, while the 2001 Mariners were +300. But if the 2018 Astros maintain their pace this season, they’ll shatter the stat of the ’98 Yankees by 53 runs.

That’s a tall order, though. We’ve seen teams run this hot at the beginning of the season, only to slow down as things progress. Twenty-five games into the 2016 season, the Chicago Cubs were on pace for a downright silly +576 run differential. That didn’t hold—they finished the season with a good, but not historic, +252—although it was good enough for the team to end a 108-year drought.

Over the past five years, two teams that led the majors in run differential went on to win the World Series—the 2013 Red Sox and the 2016 Cubs—while one team, the 2014 A’s, could have used an edge in close games, getting eliminated in a 9-8 loss in the Wild Card playoff to the pennant-winning Royals (a meager +27 run differential in the regular season). Since the turn of the millennium, only five teams that led the league in run differential went on to win the World Series.

It’d be fun to see the 2018 Astros make history and follow-up their impressive 2017 championship season with one that shatters records. Most fans, though, would be happier to see the ‘Stros go back-to-back in World Series than repeat the achievement of the 2001 Mariners, setting an American League record for wins and breaking +300 in run differential only to get clobbered 4-1 in the ALCS playoffs.

Ultimately, the pace with which the Astros are outscoring their opponents bodes well for the team’s chances—not because a great run differential guarantees anything, but because past performance likely indicates future success. There are occasionally teams that make it into the postseason because they won a bunch of close games, and got blown out in a number of their losses—the 2007 Diamondbacks, with a -22 run differential, made it all the way to the NLCS—but generally speaking, scoring many more runs than an opponent is the surest sign that a team is able to compete for the long haul. Once a team makes it into the postseason, meanwhile, the metrics by which we measure greatness tend to change.

During the regular season, teams play about half their games against good teams and half their games against bad ones. An ability to beat up on bad teams isn’t worth much when all of the teams are good—which means that, as compelling as the Astros’ potential to claim a post-war record is, their success against the best teams in the regular season is a better indicator of how the team might fare in the playoffs. To that end, as much as we’ll be watching the team’s run differential for a mark of statistical greatness, how they fare against the Yankees and the Red Sox between now and Sunday is a better marker for how a team will perform in the postseason and may tell us more about the long-term future of the team.