WASHINGTON — New sport utility vehicles and pickups are less likely to roll over in a crash than ever before, according to federal tests released yesterday, but safety advocates question how much the tests can do to reduce rollover crashes.

The National Highway Traffic Safety Administration also said automakers are selling more SUVs with electronic stability control, a system that has been shown to sharply reduce accidents. It may become a requirement in future models.

While rollovers are relatively rare — accounting for less than 3 percent of all crashes — they cause a third of all traffic-related deaths, killing 10,411 people in the U.S. in 2004, according to the most recent federal data.

NHTSA's rollover ratings derive from a calculation based on the distance between a vehicle's wheels and the height of its center of gravity. NHTSA says that formula, known as the static stability factor, accurately predicts the chances of a rollover in a single-vehicle crash.

Under its system, a five-star vehicle has a less-than-10 percent chance of rolling over, while vehicles with four stars have a 10 percent to 20 percent risk. Three stars signifies a 20 percent to 30 percent risk.

NHTSA said that of the 93 SUV models it has tested for 2006, 39 earned a four-star grade, with the remainder garnering three stars. The worst performer, the four-wheel-drive version of the Nissan Xterra, was estimated to have a 25 percent rollover risk.

Among the 59 pickup models tested so far, NHTSA said 43 earned four stars, with most others earning three stars.

The model list for this year includes corporate twins as well as results from vehicles tested in previous years and sold with no changes in 2006.

The results have shown a steady improvement over the past few years as automakers built more crossovers, or car-based SUVs, which generally ride lower than traditional truck-based SUVs.

Among SUVs, the best performers are the Chrysler Pacifica and the Ford Freestyle, at 13 percent, and the Chevrolet HHR with a 14 percent rollover risk, scores similar to passenger cars.

Other safety experts say the agency should use more than just math to predict the probability of a rollover. In the wake of the Firestone tire debacle, Congress forced NHTSA to add a driving test to its rollover measurements. That test puts vehicles through a sharp fishhook turn at between 33 and 50 mph to see if they tip up on two wheels.

For 2006 SUVs, NHTSA lists 13 as having tipped up in its driving tests, including the Ford Escape, the Chevrolet Equinox and the Hyundai Santa Fe.

But under NHTSA's system, those tests have little weight in a vehicle's final score — no more than a half-star difference — as the agency cites research that few vehicles get into rollover accidents without striking an object. The Equinox and its twin, the Pontiac Torrent, receive four stars despite tipping their wheels in tests last year.

Consumer Reports will not recommend any vehicle that tips its wheels in NHTSA's tests.

Joan Claybrook, president of Public Citizen, notes that under NHTSA's rules vehicles get extra points for passing the handling test, but aren't penalized for failing it.

"It's very misleading to the public," Claybrook said. "Tip-up has always been considered a precursor to rollover. ... If you have several models with four stars and a tip-up, there's something wrong with the test."