New crash tests on mid-size SUVs by the Insurance Institute for Highway Safety (IIHS) further undermine two popular car-buying myths: that SUVs’ size and height automatically make them safer than passenger cars, and that all side airbags are created equal.

IIHS—a research group funded largely by insurance companies—tested the Chevrolet Blazer/GMC Envoy/Isuzu Ascender/Saab 9-7X quadruplets, the Ford Explorer/Mercury Mountaineer twins, Jeep Grand Cherokee and Toyota 4Runner. Researchers also looked at different versions of the Nissan Pathfinder and Xterra, with and without optional side airbags. They subjected all vehicles to three separate evaluations. Frontal-offset crash tests, conducted at 40 mph (a slightly higher speed than the government’s full-frontal crash tests), simulate a head-on collision in which the driver swerves at the last moment. Side-impact crash tests simulate a pickup or SUV “T-boning” the test vehicle at 31 mph. And rear-crash evaluations attempt to predict how well the seats and headrests prevent whiplash.

All of the models earned a “good” rating on the frontal-offset tests, except for the Blazer/Envoy/Ascender/9-7X, which was “average.” Real differences showed up in the side-impact crash tests, however. Without side-curtain airbags, both the Pathfinder and Xterra earned only a “marginal” rating. So did the Grand Cherokee and Trailblazer models—even though they are installed with side-curtain airbags.

The problem lies in the design of the Grand Cherokee and Trailblazer’s side airbags. “Nearly every car with side airbags has both head and torso airbags, but the Grand Cherokee and Trailblazer have head protection only,” notes David Zuby of IIHS. The lack of torso protection, combined with weak side structures, resulted in high forces on the crash-test dummies and lower-than-expected scores.

Equipped with optional head and torso side airbags, however, the Pathfinder and Xterra earned “good” ratings, along with the Toyota 4Runner. The Ford Explorer rated “average.”

The poor performance of some of these SUVs surprised even the IIHS experts. “SUVs should have an advantage in side crashes because the driver and passengers ride higher up than they do in cars,” Zuby says. “People often think they’re safer in one of these vehicles, but many cars hold up better than do some of these mid-size SUVs in this test.” For complete results of mid-size SUV crash tests, see www.iihs.org.