NEW YORK (CNN/Money) - If you're shopping for a new sport utility vehicle and you want to buy one that's less likely to roll over in a crash, the National Highway Traffic Safety Administration's five-star rollover resistance ratings can be helpful.

Unfortunately, they can also be very confusing.

NHTSA's five-star rollover resistance rating system has, for practical purposes, become a two-star system. Of the 28 SUVs rated so far, all but one have either three or four stars, and 20 of those are in the three-star category.

Five stars means a vehicle has less than a 10 percent chance of rolling over in a single-vehicle crash. No SUV rated so far has attained such a rating.

Four stars means a 10 to 20 percent chance; three stars, 20 to 30 percent; and two stars, 30 to 40 percent. One star would mean a greater than 40 percent chance of rolling over in an accident, but no vehicle has scored that badly yet.

Since all SUVs fall into such a narrow range, does that mean they're all about the same? Not really. But it would be helpful to have more information about data behind the ratings and how they are calculated.

Unfortunately, the underlying data that NHTSA uses to assign rollover star ratings -- which could help you make a more informed judgment -- is difficult to find if you don't know just where to look. And there's no way, using NHTSA's SaferCar.gov site, to easily compare one SUV's stability factors to another's.

NHTSA's rollover resistance rating is based on two factors. By far, the more important of the two is a straightforward mathematical calculation based on the vehicle's dimensions.

Rollover resistance data • Data behind NHTSA Rollover Resistance Ratings

The so-called Static Stability Factor is computed by comparing a vehicle's track width with the 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 midpoint of the vehicle's mass, the point from which there is an equal amount of weight in front and behind, above and below, etc.

The stability factor is generally a number between 1.0 and 1.5. The higher the Static Stability Factor, the more stable the vehicle is and the less likely it is to tip over in a crash.

What the stars mean Star rating  Stability factor range  Chance of rollover  5  1.44 to 1.55  Less than 10 percent  4  1.18 to 1.44  10 to 20 percent  3  1.08 to 1.17  20 to 30 percent  2  1.02 to 1.07  30 to 40 percent  1  0.95 to 1.02  Greater than 50 percent   * Ranges are for vehicles that do not tip in dynamic test. Chances of rollover are for single-vehicle incidents.  Source:  National Highway Traffic Safety Administration

Among SUVs that have been rated so far, the one with the highest stability factor is the Honda Pilot with 1.25. The Pilot got a four-star rollover resistance rating from NHTSA.

There are plenty of passenger cars with four-star rollover resistance ratings. Four-star cars have rollover resistance ratings up to 1.45, though, so even the most stable SUVs are at the bottom of the 4-star range.

"That is a wide range," said Mark Krawczyk, a NHTSA communications specialist.

NHTSA is working on a system that will make it easier for consumers to compare vehicles within the same star-rating range, according to public documents.

This year, NHTSA also added a driving test to the calculation. Called the dynamic stability test, the results of this much-publicized test actually have only a tiny effect on the star rating.

Update This story was originally published in June, 2004. Since then NHTSA has made changes to the way it displays rollover ratings. Those changes address some of the issues raised in this story. For more on the changes, read our story.

The reason for that, said Krawczyk, is that only five percent of rollover accidents happen because someone turns the vehicle too sharply at high speed.

Ninety-five percent of rollovers are what auto safety experts call "tripped" rollovers, he said. That means the vehicles strikes something low, such as a curbside or shallow ditch, which tips it over.

That's the type of situation the Static Stability Factor is designed to measure and that's why it carries so much more weight in NHTSA's rollover rating system. Driving tests simply don't reflect the way rollovers happen in the real world.

In NHTA's driving test, a vehicle is driven though a left-then-right turning maneuver at speeds ranging from 35 to 50 miles per hour. Steering and speed are controlled by a computer so that the tests are precisely the same for every vehicle, and the vehicles are equipped with long outriggers to prevent them from actually rolling over.

In terms of its effect on the star rating you see on NHTSA's Web site, the test is considered strictly pass/fail. Failing has no effect on the star rating. If the vehicle tips, the rating stays the same as if it were based on the Static Stability Factor alone.

If the vehicle does make it through the maneuver without tipping, that's worth about half a star, which could mean a promotion to a higher star rating.

To make it easier to compare one SUV's rollover rating to another, CNN/Money has collected NHTSA's star ratings, static stability factors and whether or not the vehicle tipped in NHTSA's driving test and put that data together in one table. Click here to see our table.

The table includes all SUVs to which NHTSA has assigned a rollover resistance rating. More SUVs will be added to the table as they are rated.

R. David Pittle, senior vice president for technical policy for Consumer Reports, has a simple recommendation for those looking at an SUV.

"I would never recommend a car that tips up on that test," he said.

Rollover wrecks accounted for 3 percent of crashes in 2002, but 33 percent of fatalities, according to NHTSA.

That still leaves you plenty to choose from. Only seven SUVs, all of them Ford and General Motors products, have tipped on NHTSA's test.

Ford has said, in response to past inquiries about some of its SUVs tipping on NHTSA's test, that the company feels NHTSA's test does not reflect real-world evasive driving techniques and that Ford's SUVs are safe. In the most recent test, the Ford Explorer 4x2 didn't tip until nearly 50 miles per hour, said Ford spokeswoman Kristen Kinley, and then only as it was slowing down coming out of the maneuver.

NHTSA has made it clear, however, that they feel any vehicle that fails to make it through the test at 50 miles per hour has failed.

Jim Schell, a spokesman for General Motors, said while GM is pleased that NHTSA has added a dynamic test to its rating system, consumers should remember that the test is very severe. GM believes that its vehicles are safe and stable, based on real-world experience, he said.

After looking at the driving test results, consumers should compare the vehicles' static stability factors and choose the one that's higher, Pittle said.

Remember that most rollovers accidents are "tripped," and driving tests aren't a good predictor of safety in that type of situation. What you really want is the highest Static Stability Factor you can have.