2009 Chevrolet Malibu vs 1959 Chevrolet Bel AirEnlarge Photo
3) This Year's Ratings Don't Compare To Last Year's
The former NHTSA tests were based on estimated risk of serious injury in the federal government's 35-mph fixed-barrier frontal offset and 38.5-mph side-barrier tests. However, under that former system, the vast majority of vehicles tested were honor-roll students, earning four- and five-star ratings—and it was tough to tell the best from the better.
The revised test method corresponds to relative risk, using a baseline, compared to other vehicles within a class, with average running around three stars, so it assures that the agency will use more, if not all, of the ratings spread. Three stars is now "average to greater than average," while four stars is "less than average to average" and five stars is "much less than average." Ratings of one and two stars, of course, correspond to greater-than-average risks of injury.
The tests themselves are different, too, with different test dummies, test conditions, and more data. Most significant is the inclusion of a fifth-percentile female dummy to consider injuries for smaller occupants and teens. The other most significant change is the introduction of a side pole test, which simulates a side skid collision with a tree or utility pole—although that test isn't yet figured into the Overall Vehicle Score.