In the side test, the Jeep Liberty, Jeep Wrangler, Dodge Nitro and Kia Sorento, all 2008 models, were the worst performers. The Liberty, Wrangler and Nitro all received the second-lowest rating "marginal," while the Sorento had the lowest rating of "poor."

Joe Nolan, the Insurance Institute's senior vice president, said in the report that the side impact evaluation revealed some "surprising" results.

"SUVs should have an inherent advantage in such [side] crashes because drivers and passengers ride higher up than in cars. People often think they're safer in an SUV, but many cars perform much better in our side test than some of the SUVs in this group."

The IIHS report noted that the Jeep Wrangler was tested without its optional combination head and torso side airbags. Manufacturers may request a second test with the airbag option at their own expense. But Chrysler, which makes the Wrangler, did not request another test, according to the report.

"We assume that Chrysler doesn't expect the Wrangler to perform much better, even with the optional airbags," Nolan said in the report.

Max Gates, a Chrysler spokesman, said in a statement that the IIHS report contains "unsupported speculation" about the company's reasons for not paying to conduct a second side impact test for the Wrangler.

"In fact, our internal testing indicates there is additional protection provided by side air bags," he said.