The National Highway Traffic Safety Administration will begin a new rating system that predicts the probability that a vehicle will roll over in a severe crash. The new system will let consumers compare a vehicle with similar vehicles, NHTSA reported Monday.NHTSA will assign scores to supplement results based on test track performance and a mathematical formula, spokesman Rae Tyson said. The agency starting Monday will rank from best to worst cars, SUVs, minivans or pickups, he said.A five-star rating is the highest, meaning the vehicle's chance of rolling over is 10 percent or less. A two-star rating means a vehicle's chance of rolling over is between 30 and 40 percent. Here are the best and worst performers among 2004 vehicles tested so far.Passenger Cars Best: Mazda RX-8, Five stars, 8 percent Worst: Subaru Outback wagon, Four stars, 15.5 percent Pickup trucks Best: Chevrolet Silverado 4x2, Four stars, 15.9 percent Worst: Toyota Tacoma 4x4, Three stars, 28.3 percent Sport Utility Vehicles Best: Chrysler Pacifica 4x4, Four stars, 13 percent Worst: Ford Explorer Sport Trac 4x2, Two stars, 34.8 percent Vans Best: Nissan Quest, Four stars, 12.1 percent Worst: Toyota Sienna, Four stars, 15.9 percent Source: National Highway Traffic Safety Administration.