WASHINGTON — General Motors Corp.'s 2001 Pontiac Aztek received four stars out of a possible five in the latest U.S. government rollover rankings, becoming the first sport-utility vehicle to earn more than three stars.
Ford Motor Co.'s 2002 Explorer and Mercury Mountaineer sport- utilities got two stars for two-wheel drives and three stars for four-wheel drives, the National Highway Traffic Safety Administration said.
DaimlerChrysler's 2002 Jeep Liberty received two-stars for both two- and four-wheel drive models, as did Nissan Motor Co.'s 2001 Xterra two-wheel drive vehicle.
The ratings are based on a mathematical formula using the vehicle's center of gravity and width.
Some four-wheel drive models have lower centers of gravity, improving their scores in the tests over two-wheel drive models from Ford and other auto makers, NHTSA spokesman Tim Hurd said.
The rankings, based on a center-of-gravity equation, gained attention last year as the U.S. investigated fatal accidents related to Firestone tires, most mounted on Ford Explorers.
Auto makers say that the calculations don't necessarily reflect a vehicle's likelihood to roll over in real-world conditions.