SAN FRANCISCO (AP) - Jim Harbaugh took a moment and reminded his team to stay humble, to keep working. Since the day he became 49ers coach back in January, Harbaugh has talked about the ``process' of turning around a franchise.

So far, is has happened fast for San Francisco. The NFC West-leading Niners are making strides each week with Harbaugh in charge.

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Alex Smith threw two of his three touchdown passes to Vernon Davis, Frank Gore ran for a touchdown in his second straight 100-yard rushing game and San Francisco whipped the Tampa Bay Buccaneers 48-3 on Sunday for its first 4-1 start in nine years.

``I love the fact that he motivates us and has us ready to win every game,' wide receiver Michael Crabtree said of Harbaugh. ``It's like the will to win. It's not just focusing on one person. Everyone has the same goal in mind.'

And San Francisco is getting key contributions from all phases: offense, defense and special teams.

Carlos Rogers returned an interception 31 yards for a touchdown and tight ends Davis and Delanie Walker each caught TD passes of 20-plus yards in San Francisco's third straight win since blowing a late lead in a Sept. 18 overtime loss to the Cowboys.

This marked the Niners' largest margin of victory since they beat the Los Angeles Rams 48-0 on Dec. 27, 1987 - and it matched the Bucs' worst loss ever. San Francisco also scored its most points since putting up 50 against Arizona on Dec. 7, 2003.

``It's been a long time,' Davis said. ``To be ahead like we were today, I don't remember the last time I've seen anything like that.'

But a late right leg injury to wide receiver Joshua Morgan put a damper on what had been a near-perfect day.

While the Niners (4-1) beat Seattle and Cincinnati, the past two weeks are more telling: a 24-23 come-from-behind win at Philadelphia last Sunday followed by this lopsided affair against the former NFC South leader.

``He told us to stay humble and keep working,' Gore said of Harbaugh's postgame message.

Josh Freeman never found the flow that made him so effective in the rout here last November and the Bucs (3-2) were a step behind in a short week after beating the Colts on Monday night.

``Flat start. Flat finish. Flat middle. Everything,' coach Raheem Morris said. ``We didn't even get off the plane this time.'

Tampa Bay lost its cool, too - and lost a share of the South lead in the process.

A testy Morris received an unsportsmanlike conduct penalty with 9:05 left in the third quarter for yelling in the face of line judge Mark Steinkerchner.

Just more than five minutes later, safety Sean Jones had a late hit to Kendall Hunter's head after the play was over.

Linebacker Dekoda Watson was flagged for an unsportsmanlike penalty 4 seconds into the fourth quarter that set up Davis' second TD catch.

``They were really frustrated. I felt like they gave up,' Davis said. ``I was telling Ronde (Barber), 'Get your guys, get your guys, y'all got to play harder.''

Smith never got a chance to help during an embarrassing 21-0 shutout home loss to Tampa Bay last year. He watched from the sideline as former coach Mike Singletary chose Troy Smith to start instead.

He made his mark this time, going 11 for 19 for 170 yards and no sacks or interceptions before giving way to rookie backup Colin Kaepernick midway through the fourth.

The 49ers are 4-1 for the first time since 2002, the last time the franchise had a winning record and reached the playoffs - four head coaches ago.

Next up is another tough test: a Week 6 road game at Detroit, with the Lions unbeaten heading into Monday night's matchup with the Bears.

San Francisco came out of halftime with an 80-yard, seven-play scoring drive capped by Davis' 23-yard TD grab. Walker caught a 26-yard pass for the 49ers on their opening drive of the game.

``That's what we can do every single week,' left tackle Joe Staley said.

Rogers' third interception of the season, on a pass intended for Kellen Winslow early in the second quarter, stopped any momentum Tampa Bay might have gained after Gore fumbled deep in Bucs' territory.

``When you put points on the board on defense like that, it takes a lot out of an offense,' Rogers said.

Rookie Chris Culliver thwarted the next Buccaneers' drive with his first career interception.

Dashon Goldson provided a nice block on Freeman as Rogers stepped into the end zone on the right side, then Goldson forced a fumble by Mike Williams late in the third that was recovered by Ray McDonald.

Freeman went 17 for 33 for 187 yards with two interceptions and two sacks.

``It's surprising. How could it not surprise you?' Bucs center Jeff Faine said.

Notes: Freeman's six INTs this season equal his 2010 total. ... Gore's scoring run gave him 38 rushing TDs for his career, fourth on the franchise list. ... Adam Snyder started at RG for the Niners for the second straight week.