For the first time, some truck-based sport utility vehicles received as many as four out of five stars in rollover ratings, according to 2004 model ratings released yesterday by federal regulators. But the higher ratings may not necessarily mean the vehicles have become safer, because the government has changed the way it tests them.
          In the new tests, using real vehicles on a test track as well as mathematical calculations, the four-wheel-drive versions of several similar sport utility vehicles made by General Motors -- the Chevrolet TrailBlazer, GMC Envoy, Oldsmobile Bravada and the new Buick Rainier -- earned four stars, even though they are virtually identical to G.M. vehicles that rated only three stars last year.
          The Volvo XC90, a car-based S.U.V. that has been promoted by the Ford Motor Company, Volvo's parent, for its aggressive use of rollover-prevention technology, also received four stars, not the government's highest rating of five stars - a result that disappointed the company.
          Two pickup trucks, the Toyota Tacoma and the Ford Explorer Sport Trac, tipped up on two wheels during the tests, a poor showing in a rollover test. Regulators also said they were reviewing test results on two Ford S.U.V.'s, the Explorer and the Mercury Mountaineer, but did not say why.
          Vehicle rollovers kill more than 10,000 Americans every year. They are rare but deadly, occurring in fewer than 3 percent of accidents but accounting for a third of vehicle occupant deaths. And the problem has been on the rise as sales of S.U.V.'s have boomed; sport utilities have higher centers of gravity than cars and are three times as likely to be involved in rollover deaths.
          Congress ordered regulators to come up with a more rigorous rollover test in 2000, after nearly 300 rollover deaths in Explorers equipped with Firestone tires. The National Highway Traffic Safety Administration released the results of these new tests on 16 model lines yesterday. The results were not necessarily indicative of improvements from the 2003 model year, but rather showed that the new test would provide different results and new information about vehicles that had trouble negotiating sharp turns on a test track. In the new tests, actual vehicles are also driven through as many as 10 fishhook maneuvers, a jarring series of turns intended to replicate what happens when drivers drift off the road and then try to overcompensate in their steering.