Editor's note: An earlier version of this story incorrectly said Ford plans to add wheel blockers to 2016 SuperCab and regular cab pickups. Ford said it is studying which "countermeasures" to add, and they will vary by cab type.

After a highly unusual follow-up crash test, the Insurance Institute for Highway Safety has concluded that Ford Motor Co. “shortchanges” some buyers of its redesigned 2015 F-150 pickups by equipping certain models with protective steel bars while leaving them off others.

In separate crashes, a four-door crew cab F-150 SuperCrew aced IIHS’s tests and earned the safety agency’s coveted Top Safety Pick rating. But an extended cab SuperCab version of the F-150 received the second-lowest rating of “marginal.”

Why the difference?

IIHS says it comes down to four protective steel bars that Ford installed on the SuperCrew that are left off the SuperCab. The tubular bars, welded to the frame and placed in the front wheel wells, also are missing from regular cab F-150s.

David Zuby, IIHS’s chief research officer, said leaving the bars off the SuperCab “shortchanges buyers who might pick the extended cab thinking it offers the same protection in this type of crash as the crew cab.”

“It doesn’t,” he said.

“In a small-overlap front crash like this, there’s no question you’d rather be driving the crew cab than the extended cab F-150.”

In an e-mailed response to questions from Automotive News, Ford spokesman Mike Levine said Ford will add "countermeasures to the SuperCab and Regular Cab for the 2016 model year. The type of countermeasure and structure will vary by cab type."

Changing course

The IIHS originally tested the SuperCrew in April under its usual procedures to test only the highest-volume version of vehicle nameplates. The researchers were not planning to test any other F-150s.

The institute changed course after learning from Automotive News that the SuperCab lacks the SuperCrew’s wheel blockers, the term for the protective bars used by the IIHS and many engineers.

The IIHS, a 55-year-old nonprofit funded by auto insurers and based in Arlington, Va., plans to release the results of the new tests Thursday. Automotive News obtained an advanced copy of the report through another source and was granted access to IIHS’s chief research officer.

“Based on these two tests, those structures which extend from the outboard edges of the frame rails seem to do a good job of protecting the occupant compartment from excessive intrusion in this type of crash,” Zuby said.

Wheel protectors

As Automotive News first reported, SuperCrew F-150s come standard with a pair of what Ford calls “protectors” in front of and behind each front wheel. The tubular steel bars extend out from the frame, absorbing energy in a frontal impact and deflecting the wheel away from the passenger cabin.

The 2015 models without the protectors -- the SuperCab and regular cab pickups -- historically account for about 25 percent and 5 percent of total F-150 sales, respectively, according to a Ford spokesman.

The IIHS already had tested the SuperCrew F-150 in April when Automotive News informed it that only the SuperCrew had the protective bars.

The revelation prompted the agency to delay a planned report on F-150 crash safety and to conduct a second test, on June 11, this time on a SuperCab.

In May, when Automotive News asked a Ford spokeswoman why the protectors were on the crew cab but not other versions, she said, “It’s something that we do regularly. It’s to optimize the structure. All of that is to meet regulatory requirements and achieve public domain ratings.”

Asked to clarify what public domain ratings were, she replied, “crash testing.”

Spokesman Levine said Ford had no plans to recall 2015 F-150s without the blockers, which the company also calls Small Offset Rigid Barrier countermeasures.

"We are evaluating which specific changes we will make to the SuperCab and the Regular Cab to improve performance in the IIHS small-overlap front crash test," he said. "It is important that any changes do not compromise performance on other crash tests."

Levine added: “The 2015 F-150 is the safest F-150 ever. It is the only full-size, light-duty truck to earn NHTSA’s highest five-star rating for the driver and passenger for all crash test modes and cab configurations. In addition, the 2015 F-150 SuperCrew is the first large pickup in the industry to earn an IIHS Top Safety Pick in the current rating system.”