NEW YORK (CNN/Money) - When you read National Highway Traffic Safety Administration's new dynamic stability test, you might think that sport utility vehicles are doing better than expected in that driving test.+

In fact, since NHTSA introduced that test, TK vehicles have seen their 5-star resistance ratings go up compared to their ratings before the new test was introduced. Before, SUV rollover resistance was based solely a vehicles' dimensions rather than any actual driving test.

On its Website and in its press releases, NHTSA says the new results of the new dynamic test are combined with the calcution based on the vehicle's dimensions.

According to NHTSA's formula, the government auto safety agency's new dynamic stability test could only improve a vehicle's rating. Under no circumstances, no matter how poorly a vehicle handles in NHTSA's driving test, will its rollover resistance rating ever go down.

In response to concerns about the tendency of sport utility vehicles to roll over in crashes, a particularly deadly type of wreck, NHTSA began adding star ratings for rollover resistance to its other crash test results in 2001. Until this year, the rollover resitance ratings were based solely on a number called the Static Stability Factor. That figure, a number that generally falls somewhere between 1.0 and 1.5, is computed based on the track width of the vehicle and height of its center of gravity.

A vehicle's track width is the distance between the centers of its right and left tires. The center of gravity is the center of the vehicle's mass, the point from which there as in equal amount of weight in front and behind, above and below, etc.

The higher a vehicle's center of gravity and the narrower its track width the less stable it will be in turns. SUVs are taller than cars and generally have higher Static Stability Factor numbers than passenger cars, indicating a greater tendency to tip over.

NHTSA uses that figure to give a one-to-five star figure based on the vehicle's calculated likelihood of rolling over in a single-vehicle crash.

The Insurance Institute for Highway Safety used federal crash data to compute the likely ranges of static stability factors for each star rating.

The SSF factor is a reasonable basis for judging the stabiltiy of a vehicle, according to experts in motor vehicle dynamics interviewed by CNN/Money, including an David Renfroe, an engineer who has testified against Ford Motor Company in rollover-related lawsuits and Barry Parker, a former Idaho State University physics professor and author of the book "The Isaac Newton School of Driving: Physics and Your Car."

Still, many consumer groups, as well as representatives for the auto industry, felt that rating system was insufficient. For one thing, actual rollover tendency can be affected by things like the vehicle's suspension system and whether it has technology like electronic stability control which helps correct for overly aggressive steering maneuvers.

So, after looking into a variety of testing regimens, NHTSA instituted a new dynamic driving test. The results of that test, in which the vehicle is run through a computer-controlled left-then-right turning maneuver at different speeds, are combined with the SSF to create a new five-star roll over resistance figure, NHTSA says. For safety, vehicles in the test are fitted with long outriggers that prevent from them tipping completely,

In its press releases and on it's public Website, NHTSA does not describe how the driving test are used to compute the final rollover resistance rating. The word "combined" could lead you to think that performing poorly on the driving test would cause a vehicle's star rating to come down.

Actually, NHTSA assumes vehicles will perform poorly on its dynamic driving test so there is no penalty for tipping over. If a vehicle tips over, its star rating is based on its SSF numbr alone.

If the vehicle stays on all four wheels during the driving test, it gets an extra half star. (Since there are no half-stars in the final ratings, that only makes a difference for vehicles that were already close to higher star rating based on the SSF alone.)

The Ford Explorer, which had 3 stars based on its SSF score prior to the dynamic stability test, still had 3 starts after the dynamic stability test. The fact that it tipped up during the test was pointed out in a NHTSA press release announcing the new ratings, but the fact that tipping had had no effect on the rating was not mentioned. (CHECK THIS).

The fact that the Exporer tipped during testing is also not pointed out on in the tables of safety ratings on NHTSA's Safercar.gov Website, where the public can view NHTSA's safety ratings of vehicles. Nor is that fact noted for any of the other vehicles that tipped during testing.

NHTSA runs each vehicle through the test maneuver at 35, then 50, miles per hour. If the vehicle tips, the vehicle is then run through the maneuver at various speeds until the highest speed at which it can safely make the maneuver is determined.

However, NHTSA does not reveal the results of those tests to the public.