The government's traffic safety agency is expanding its rollover rating system (search) for cars and trucks.

The National Highway Traffic Safety Administration's (search) old rollover ratings were based on height and width as well as a test that includes a sharp turn at up to 50 mph. Five stars are given to vehicles that roll over 10 percent of the time or less, and one star to vehicles that roll over between 40 and 50 percent of the time.

The new system, available Monday on NHTSA's Web site, retains the star rating but also lets consumers compare a particular vehicle's grade to the ratings of similar vehicles. The system also shows consumers the percentage chance the vehicle would roll over in a crash similar to NHTSA's test.

For example, NHTSA's system gave the 2004 Chrysler Pacifica sport utility vehicle four stars, and showed that ratings for SUVs generally ranged from one to four stars. The graph also shows the Pacifica's percentage chance of rollover — 13 percent — was the lowest of all SUVs.

NHTSA gave Ford's front-wheel drive Explorer Sport Trac the worst rollover rating of any SUV and said its chance of rollover was 34.8 percent.

So far, the system only rates vehicles from the 2004 model year.

Rollovers represent only 3 percent of all crashes, but they are especially severe. They are responsible for one-third of the 43,200 deaths on U.S. highways each year, said NHTSA Administrator Dr. Jeffrey Runge. That's partly because of vehicle engineering and partly because people aren't using seat belts, Runge said, noting that three-quarters of those who die in rollovers aren't belted.

Runge said the new system was designed to make NHTSA's ratings more user-friendly. He encouraged consumers to consider the rollover rating when they're buying a vehicle.

"If no one buys vehicles that are prone to roll over, then manufacturers will stop making them," he said.

General Motors Corp. and Ford Motor Co. said NHTSA's moving rollover test is extremely severe and representative of only about 5 percent of actual rollovers.

"We believe that in real-world driving situations, these vehicles are stable and resist rollover," said GM spokesman Jim Schell.

Ford spokeswoman Kristen Kinley said data indicate Ford vehicles are more rollover resistant than NHTSA suggests.

NHTSA spokesman Rae Tyson acknowledged the rollover test mimics a rare type of crash. But he said a vehicle's tendency to roll over can be predicted accurately based on its height and weight, which is a significant part of the agency's rating.

The ratings do not necessarily consider any equipment designed to prevent rollovers, such as electronic stability control systems. NHTSA only tests vehicles with stability control if it is a standard feature or if more than half of consumers choose it as an option. Stability control systems are now available on about 10 percent of vehicles in the United States.