MELVILLE, N.Y. — An insurance group is warning that although most small cars offer good protection in frontal impacts, many fall short if they are hit in the side or rear.

The Insurance Institute for Highway Safety says in a report released Wednesday that only three of nine popular small cars tested, the Suzuki SX4 and the Pontiac Vibe and mechanically related Toyota Matrix, were rated “good” in protecting passengers in frontal and side crashes.

But the Suzuki was deemed “marginal” in rear impacts.

The institute’s rear-impact test evaluates the protection provided by seat backs and head restraints against whiplash neck injuries. The Matrix and Vibe’s rear-impact protection hasn’t yet been evaluated.

The institute’s report comes as this year’s earlier $4-plus-a-gallon gasoline prices combined with the weak economy to stimulate interest in small cars. Through November, for example, compacts like the Honda Civic and Ford Focus accounted for almost 16.7 percent of new-vehicle sales in the United States, up from 13.3 percent a year earlier, according to market researchers J.D. Power and Associates.

The side-impact test, usually done with air bags functional, simulates the vehicle being struck at 31 mph by an SUV. An easier federal government test simulates the vehicle being struck by a passenger car.

“In a good-rated vehicle, a driver could walk away from the side-impact crash without serious injury,” institute spokesman Russell Rader said. “In a poor-rated side-impact vehicle, we would expect a high likelihood of life-threatening injuries.”

The lowest-ranked vehicle overall in this group was the Chrysler PT Cruiser, which performed well in the 40 mph frontal crash test but was deemed “poor” by the institute in its side- and rear-impact protection.

In a statement, Chrysler noted that the Cruiser earned four out of a possible five stars for the driver and five stars for rear occupants in the federal government’s side-impact test. “Chrysler, Jeep and Dodge vehicles meet or exceed all applicable federal safety standards,” the statement read.

Ratings for all cars tested by the institute are at iihs.org.

How they rated

Crash ratings of nine new cars tested by the Insurance Institute for Highway Safety

G: Good

A: Acceptable

M: Marginal

P: Poor

NR: Not rated

Vehicle name / Front / Side / Rear

Suzuki SX4 / G / G / M

Toyota Matrix / G / G / NR

Pontiac Vibe G / G / NR

Ford Focus G / A / G

Chevrolet HHR G / A / M

Hyundai Elantra G / M / A

Saturn Astra G / M / A

Chrysler PT Cruiser G / P / P

Mini Cooper G / A / G

Source: Insurance Institute for Highway Safety