The ground-hugging Mazda RX-8 sports car ranked best and the big Ford Explorer Sport Trac 4x2 truck was worst in the government's new rating of rollover safety for 2004 model-year vehicles, released yesterday by the National Highway Traffic Safety Administration.

It was the first model year the government has evaluated rollover safety using an actual driving test in addition to simply computing a vehicle's center of gravity. The agency has released some of the results in past months, but yesterday completed the full slate of vehicles it planned to test this year and augmented the ratings with new information not previously made public.

The new information went beyond the usual star rating system for evaluating performance -- one star is worst, five is best -- to also provide the percentage likelihood that each car or truck might tip over in a single-vehicle crash.

The little RX-8, wide and low to the ground, was the only vehicle to earn a five-star rating and had the lowest rollover potential at 8 percent. The Explorer Sport Trac 4x2, on the other hand, earned the only two-star rating, had a nearly 35 percent chance of rollover and tipped up during the driving test.

"Consumers should . . . make buying choices accordingly," NHTSA chief Jeffrey W. Runge said in a news conference. He added that the agency is not telling buyers to stay away from low-rated vehicles such as the Explorer, noting that "there are lots of vehicles out there that are prone to roll over and we call them trucks."

The many thousands of people who choose to buy such vehicles should know that they must be driven with extra care, he said, and the rollover rating "should be part of the [decision-making] formula, as well as price."

The full list of vehicle rollover ratings can be viewed on the NHTSA Web site www.safercar.gov. It does not contain every make and model on sale in 2004, but features many of the most popular types. For instance, there are entries for new vehicles such as the Chrysler Pacifica, a cross between a wagon and a sport-utility vehicle that earned four stars and showed a 14 percent rollover rate for the two-wheel-drive model and 13 percent for all-wheel-drive.

Car safety advocates have criticized NHTSA's rating system for being too broad, and Runge said the enhanced ratings are partly an effort to address those concerns. The percentages highlight the problem with the star ratings; the Toyota Tacoma 4x2 pickup showed a 20 percent rollover likelihood and tipped up during the driving test, but still earned the same number of stars -- four -- as the Acura TL sedan, which did not tip and had only a 10 percent chance of rollover.

Safety groups also complain that NHTSA puts too much emphasis on evaluating a vehicle's center of gravity and does not give enough weight to the driving test. Yesterday, agency officials defended their approach, pointing out that 95 percent of all rollover crashes involve a car or truck being "tripped" by a rock or some other roadway obstacle. Only 5 percent of real-world rollovers are caused by the type of sharp turning maneuver used in NHTSA's test, which was mandated by Congress.

Runge said the agency is studying new tests that would rate vehicles for handling abilities, as well as tests of new electronic stability control systems, and will consider releasing those results as a further supplement to rollover ratings.