Ford Motor Co.'s (F) sport utility vehicles, including some models of the best-selling Explorer and the Mountaineer, are among the worst on the road for rollover risk, the government said Monday.

The Explorer Sport Trac (search) two-wheel drive posted the single worst rating for rollover propensity among all 2004 vehicles analyzed -- including cars, vans and SUVs -- in updated safety ratings released by the National Highway Traffic Safety Administration (search).

The Explorer Sport Trac four-wheel drive, the Explorer four-door, two-wheel drive, and the Mountaineer four-door, two-wheel drive were in the bottom six of the SUV class, which as a group posted the lowest scores.

The Mercury Mountaineer (search) is the corporate twin of the Explorer.

Safety experts concluded long ago that sport utilities are much more prone to roll than passenger cars in single-vehicle crashes, but the government's five-star safety ratings were criticized as incomplete and vague.

To address that concern, NHTSA for the first time has assigned a percentage risk for rollover based on a mathematical calculation of a vehicle's measurements and a road test that includes extremely sharp turns.

Most cars did better than the highest-ranked sport utility for rollover safety.

The safest vehicle overall in the latest ratings, the Mazda Motor Corp. (search) RX-8 four-door, has an 8 percent chance of rolling over. By comparison, the two-wheel drive Explorer Sport Trac has a nearly 35 percent chance, the government said.

Ford, in response to the updated government ratings, said on Monday that its own data show Explorer models perform the same as or better than other vehicles in the same class.

"We're trying to work through the data and see how NHTSA's applying these numbers. While we believe the NHTSA rating system has some value, we don't believe its a good indicator of how a vehicle performs in the real world," said Ford spokeswoman Kristen Kinley.

Rollovers represent only a small fraction of crashes on U.S. roads but a quarter of all traffic deaths, which rose to 43,000 in 2003, preliminary statistics show. Most rollover deaths occur in single-vehicle accidents.

Rollovers accounted for nearly 40 percent of fatal accidents that involved SUVs last year. Rollover deaths in those vehicles rose by 10 percent to 2,700 in 2003, government figures show.

The new rollover ratings can be accessed at www.safercar.gov