Santa Clara, CA (KPIX)- Through six weeks of the NFL season it’s fair to say that the San Francisco 49ers are the biggest surprise of the year.

Kyle Shanahan’s team has compiled a 5-0 record and is coming off a dominant victory over the Super Bowl runner-up Los Angeles Rams last Sunday. While Shanahan’s offense has certainly played a factor in the strong start, ranking fourth in total offense and third in points per game, it has been the defense that has truly led the way.

Through five games, the Niners defense has allowed just 64 points (12.8 per game) trailing only the New England Patriots in that category. The turnaround is a bit staggering, considering that the defense allowed the fifth-most points per game last season at 27.2. What has been the biggest key? For Inside The NFL analyst Phil Simms, it’s been the continued use of high draft picks on impact defensive linemen.

“Having a couple of bad seasons, a couple of bad years in a row has really helped them, because they have drafted a couple of defensive linemen that are coming through big-time. (DeForest) Buckner, (Arik) Armstead, getting (Joey) Bosa, they have all been terrific,” said Simms. “The size and speed are a big deal in the NFL. They have both. And they have it in extremes. They have great size, great speed and they are really well coached.”

Stream your local NFL on CBS game live with CBS All Access.

In that respect, maybe we should have seen this kind of season coming, considering that there are currently four former first-round picks in the front seven, when you include Solomon Thomas along with Buckner, Armstead and Bosa. Add in key offseason additions like Dee Ford and Kwon Alexander, and defensive coordinator Robert Saleh has plenty of talent to work with. Even still, normally, when a defense adds several new guys over the course of one offseason, it takes awhile for the new pieces to gel. That hasn’t been the case here, which has stunned Simms. But, he believes that the Niners are a true contender this season.

“I’m shocked at how it has come together so quick and so well for them,” said Simms. “But, they are for real and I would not take my eyes off of them for the rest of the year.”

This week, the 49ers make the trip to the nation’s capital to face the one-win Washington Redskins. It’s a game that the Niners are expected to win by a fairly comfortable margin, with oddsmakers installing them as 10-point favorites, despite having to fly cross-country. The Redskins offensive line has been a mess this season, allowing 15 sacks and 38 quarterback hits through their first six games. With a pass rush that has already generated 17 sacks and a league-leading 10.8% adjusted sack rate, Sunday’s matchup looks like another opportunity for the defense to make a statement.

You can catch Phil Simms along with Ray Lewis, Michael Irvin, Brandon Marshall and host James Brown on Inside The NFL every Tuesday night at 9 p.m. ET/PT on Showtime.