Crash tests of some popular midsize sport utility vehicles turned in an unexpectedly mixed performance in side-impact tests, a leading insurance group said on Thursday.

The Insurance Institute for Highway Safety found some late-model SUVs performed worse than cars, a result that challenges a belief among consumers that sport utility occupants are safer in some crashes because they are heavier than cars and occupants are seated higher.

The performance of some of these models in the side test was surprising, David Zuby, who heads the crash-test program at the insurance institute.

The insurance group which conducts a range of tests on new models each year that are independent of the U.S. government's crash test program.

Automakers stood by the safety performance of their vehicles in the recent tests that illustrated the importance of side airbags that protect the torso.

Manufacturers of the best selling sport utilities, including Ford Motor Co, General Motors Corp and Chrysler LLC, have spent years making design changes and adding stability systems to address rollover risk. Improvements also have been made to boost the performance of some SUVs in frontal crash tests.

But SUV sales have recently dipped, largely because of higher gasoline prices and low fuel efficiency, and automakers are offering more car-like crossover vehicles, like the Ford Edge and the Chrysler Pacifica.

The institute noted that SUVs are safer than they were just a few years ago but none of the six vehicles in the latest round of tests earned the group's top safety rating in all categories -- front, side and rear crash protection.

People often think they're safer in one of these vehicles but many cars hold up better than some of these midsize SUVs in this test, Zuby said of the side-impact assessment.

Performance varied considerably in the group's tests that used a movable barrier to strike the side of a vehicle. The barrier simulates another SUV or a pickup traveling at 31 miles per hour.

Two Nissan SUVs, the Pathfinder and the Xterra, posted good ratings -- the highest -- but only when equipped with optional side airbags. Without the airbags, the vehicles earned marginal scores - the second worst on a four-rating scale that slides from good, to acceptable, to marginal and then poor.

The 4Runner, made by Toyota Motor Corp, equipped with standard side airbags received a good rating. The Ford Explorer with standard side air bags got an acceptable rating.

But Chrysler's Jeep Grand Cherokee and the Chevrolet TrailBlazer, one of the top SUVs made by GM, posted marginal scores. The two are equipped with standard side airbags for head protection but do not include similar technology for the torso. Side airbags are standard in more cars than SUVs.

Nissan and the other automakers expressed confidence in their vehicles and noted the models involved met or exceeded government safety standards for the same tests. The insurance institute side-impact test is considered by some experts to be tougher than the National Highway Traffic Safety Administration test that mimics a car traveling at higher speed.

A GM spokesman called the insurance industry test severe and a Chrysler statement said no single test can determine a vehicle's overall safety performance.