Of all the vehicles undergoing crash tests this year, few will be as closely watched as the new 2015 Ford F-150. That's not only because it remains the top-selling vehicle in America year after year, but also because it features an aluminum body instead of steel. While the F-150 performed well in the Insurance Institute for Highway Safety test, one factor prompted the institute to undertake a second round of testing that uncovered a problem.

Like most full-size pickups, the F-150 is available in several cab styles: the regular two-door, the extended SuperCab and the four-door SuperCrew. The IIHS typically takes the most popular version of a particular model for testing, and in the Ford truck's case that meant the SuperCrew. The F-150 performed well in all the tests the IIHS put it through, including the small overlap test in which the vehicle is driven 40 miles per hour into a five-foot-tall barrier impacting the front left corner of the vehicle. Its overall performance in the tests earned the F-150 a Top Safety Pick rating, missing out on the higher Top Safety Pick + rating only because it doesn't have an automatic braking system.

But how do the other versions of Ford's best-seller hold up? Given that even less popular versions of the F-150 still sell more than many other vehicles on the market, the IIHS put an extended cab through the same battery of tests. It performed comparably except in one area: the small overlap test. In that case, the extended cab model's steering column was pushed eight inches into the cabin (dangerously close to the crash test dummy's chest), the dummy's head missed the airbag almost entirely and hit the instrument panel, and the dummy's legs would risk sustaining "moderate" injuries.

The reason for the disparity is that "Ford added structural elements to the crew cab's front frame to earn a good small overlap rating and a Top Safety Pick award but didn't do the same for the extended cab," according to the Institute's chief research officer David Zuby. "That 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."

A Ford spokesman responded: "We addressed the IIHS small overlap front crash in our high-volume SuperCrew first, which accounts for 83 percent of 2015 F-150 retail sales. We are adding countermeasures in the SuperCab and the Regular Cab in the 2016 model year. Based on this IIHS data, both vehicles offer similar occupant protection levels." (For the full Ford statement, scroll down.) The small overlap test was first introduced by the IIHS in 2012.

As mentioned earlier the extended cab performed about the same as the crew cab in all other tests, including the moderate overlap test. The Institute says it will also subject "multiple variants" of other pickups to the same standards. In its testing, the IIHS also looked at repair costs for the aluminum bodywork on the new F-150 as compared to the previous steel-bodied model, and found that repair costs to be 26 percent higher for the new model. Ford disputes those figures based on independent data. You can delve into the results broken down in the IIHS statement below, or get the gist of it from the accompanying video clip.