The Department of Transportation unveiled revisions to the government's Five-Star Safety Rating System Tuesday, which will make it more difficult for new cars and trucks to earn top scores.

Cars will still be rated on a scale of one to five, with five being the highest.

However, under the new "Stars on Cars" system, an overall score will combine the results of all tests and compare them to the average risk of injury in other vehicles. The change also takes into account crash-prevention technologies that were not included in previous tests.

"We've raised the bars on safety," Transportation Secretary Ray LaHood said. "More stars, safer cars. People really have to prove to us these cars deserve a five-star rating."

The new rating tests will now include a simulated crash into a pole and include female crash dummies.

Only two of the 35 vehicles tested under the new standards have received a five-star rating -- the 2011 BMW 5 Series and a version of the 2011 Hyundai Sonata. The Toyota Camry, the best-selling passenger car in the U.S., received three stars.

"Stars on Cars" was started in 1979 as a way to generate interest in automobile safety equipment like side-impact air bags and anti-rollover technology.

The agency made the changes because more than 90 percent of cars tested under the old system were earning top grades, making it difficult to distinguish the best performers.