Reported on a five-star system, the rollover ratings are based on an engineering analysis of each vehicle's center of gravity and the width between the front tires.

And new safety warnings about the stability of truck-based sport utilities, especially in rollover ratings, have some people reassessing their belief that S.U.V.'s are safer.

To date, the agency's rollover ratings have been derived from mathematical equations based on factors like the distance from the vehicle's center of gravity to its wheels.

He said he would not let his children drive an S.U.V. with a low rollover rating.

According to the government's new rollover ratings, which rank vehicles from one to five stars, the changes Ford made each added a quarter star.

Last week, the government said that of the 2005 sport utility models, 24 had received four stars out of five in the rollover ratings.

Starting with 2004 models, the rollover ratings combine the results of both the static stability factor and the handling test.

(Indeed, a recently published federal safety scorecard gives a fully loaded Explorer the worst rollover rating, one star out of a possible five.)

The xD also received a four-star rollover rating, which means that there is an estimated 15 percent chance of rollover in a single vehicle collision.

The third testing component, the rollover rating, resulted in a grading of four-stars.