From the moment Coach Jim Harbaugh arrived in San Francisco in 2011, the 49ers have been one of the N.F.L.’s best teams.

In Harbaugh’s first two seasons, the 49ers ranked third in combined wins (24) and points differential (275), trailing only the New England Patriots and the Green Bay Packers in both categories. In those two seasons, San Francisco allowed 502 points, easily the fewest in the league. Even after losing in the Super Bowl in February, Harbaugh’s 49ers seemed poised to become the next dynasty. But after defeating the Packers in Week 1, San Francisco has been outscored, 56-10, in the last two weeks.

In Seattle, the 49ers were 3-point underdogs, and lost, 29-3. At home against the Indianapolis Colts, San Francisco was a 10 ½-point favorite, and lost by 20 points. For the season, San Francisco has fallen short of expectations — defined as the point spread in the game — by an average of 17.5 points a game (only the Giants, at 18.3 points, have been more disappointing). How have teams with similar expectations and results fared over the course of the rest of the season?

From 1990 to 2012, 14 teams have met three criteria: won at least 10 games in the previous year; on average, were favored to win the first three games of the next season; and failed to cover the spread by, on average, at least 10 points. On average, those teams won 12.5 games the previous season but just eight in the season in question, an indication that the slow start is a sign of mediocre things to come.