The Dolphins take their sputtering offense into San Francisco on Sunday, and this question is difficult to avoid for even the most optimistic of Dolfans:

Where are the points going to come from against a 49ers defense that allows the league’s fewest?

The Dolphins are 27th in scoring and have topped 20 points only once in their past five games. Oddly, during that stretch, Miami generated its highest point total (24) against Seattle’s fourth-rated defense.

The Dolphins mustered just 14 points against the Bills’ defense (ranked 24th), 16 against the Patriots defense (ranked 26th) and three against a Titans defense ranked 27th.

“They’re the best [defense] we’ll go against all year,” center Mike Pouncey said of the 49ers.

Coach Joe Philbin and quarterback Ryan Tannehill both made the point last week: Teams aren’t going to win a lot scoring 19 points per game, which is the Dolphins’ average and better than only St. Louis, Philadelphia, Jacksonville, Kansas City and Arizona.

And the challenge this week looks especially daunting:

• The 49ers are No. 1 in the league in yards allowed per play. The Dolphins are 25th offensively in yards per play.

• The 49ers have allowed the second-fewest rushing yards per attempt. The Dolphins are averaging more yards per rush than only 11 teams.

• The 49ers have allowed the second-fewest passing yards. The Dolphins are 23rd in passing yardage.

Philbin and offensive coordinator Mike Sherman have tinkered a bit in recent weeks — such as using tight end Anthony Fasano several snaps at fullback, running a reverse to Marcus Thigpen last Sunday, targeting Charles Clay far more than usual against Seattle and incorporating receiver Rishard Matthews into the offense the past three weeks after he wasn’t used at all the first nine.

Too many issues

But nothing has mattered in the big picture. Miami still struggles to sustain drives. Third downs have become a disaster area, with Tannehill posting the league’s lowest passer rating (60.1) and tossing six interceptions compared with just one touchdown.

And the Dolphins have been in too many third-and-longs because of an inability to crank out yardage consistently on first and second down.

“It’s a little bit of everything,” running back Reggie Bush said. “It’s red zone, it’s third down. We haven’t hit a consistent stride at any point in the season. We’ve been up. We’ve been down. We’ve been here. We’ve been there. Some games, we’re running well and not throwing it well. Some games, we’re throwing it well and not running it well.”

Here’s what is a bit disconcerting: Whereas the league’s other rookie quarterbacks have raised their play, Tannehill has had only one clearly above-average game in the past month — the sterling performance in the comeback victory against Seattle.

“I have to get the pass game going early on in games and be consistent throughout games,” he said. “Third down is an area we’re really focusing on right now. We haven’t been good the past two weeks. We’re putting our defense back in tough situations.”

Tannehill’s completion percentage (57.9) has fallen to 27th in the league, and he was exasperated after misfiring on two potential touchdown passes to Brian Hartline last week.