San Francisco ? For the first time in a taxing preseason, almost everything went right for the San Francisco 49ers.

Tim Rattay completed 16 of 21 passes and led four scoring drives in the first half as the 49ers beat the San Diego Chargers 27-3 on Wednesday night in the exhibition finale for both teams.

San Diego quarterback Doug Flutie passes during a preseason game against San Francisco. The 49ers drubbed the Chargers, 27-3, Wednesday in San Francisco.

San Francisco (2-3), banged up and thin after a brutal preseason that included a trip to Japan and three games in 10 days, improbably posted an easy victory. The Niners jumped to a 21-point lead in the first half under the direction of Rattay, whose inconsistency finally ended when he was paired with the team’s first-stringers.

Instead of capitulating to San Diego, as coach Steve Mariucci all but suggested his team might do, the 49ers’ backups and fringe players jumped all over the Chargers (2-2) on both sides of the ball in their best-looking game of the preseason.

“I was probably one of the only guys that was happy we had five preseason games,” said Rattay, who passed for 158 yards in easily his best game of the summer. “I got a ton of time out there today with our best guys, and I think I made the most of it.”

Pro Bowl tandem Jeff Garcia and Terrell Owens didn’t play for the 49ers, but Rattay finally might have solidified his job. Rattay led a 69-yard scoring drive on the 49ers’ second possession, scoring on a 2-yard run after dropping the snap from Jeremy Newberry.

“Tim Rattay has made great progress in every preseason game that we’ve played, and I’m really happy with where he is right now,” Mariucci said. “He played smart and had a lot of different experiences this preseason, so in some ways, mission accomplished.”

While Rattay solved one dilemma for the 49ers, Jamal Robertson opened up a whole new quandary.

Robertson, the NFL Europe rushing champion who’s fighting to make San Francisco’s roster, scored on a pretty 11-yard run in the second quarter. He finished with 112 yards on 21 carries in the culmination of a strong camp for a team that already has five quality backs returning from last season.

“Jamal has really made some progress,” general manager Terry Donahue said. “He’ll force us to have some really intense discussions that we didn’t anticipate having.”

Though the 49ers barely had enough healthy players to field a complete defense, they made short work of the Chargers. San Diego managed just 221 total yards 82 in the first half while Drew Brees and Doug Flutie directed the offense.

“There’s no one play that determined the outcome of this game,” Chargers coach Marty Schottenheimer said. “We were clearly outplayed. I’m disappointed in how we played.”