The Insurance Institute for Highway Safety, a testing and advocacy group funded by the U.S. auto insurance industry, said only one of eight small car models it tested had performed well in three different simulated crash situations.
The Versa from Nissan Motor Co., the largest of the eight "minicars" tested by the IIHS, was the only model to achieve the highest ratings in simulated front, side and rear crashes.
Minicars were defined as those weighing about 2,500 pounds or less. A typical midsize car weighs about 800 pounds more and a midsize sport utility vehicle weighs about 4,000 pounds more, according to the institute.
The IIHS performs more stringent crash tests than federal safety regulators and its findings tend to be closely followed by consumer groups.
Adrian Lund, president of the IIHS, said the smallest cars on U.S. roads tend to be at greater risk in collisions involving bigger vehicles because "you can't repeal the laws of physics.
"It's like putting a featherweight in the ring with a heavyweight. The featherweight tends not to do well, and it's the same for cars," he said. "Their crashes are going to be more severe."
Toyota Motor Co. had two subcompacts that were given "poor" ratings in 31-mile-per-hour side collisions with a barrier designed to perform like the front-end of a pickup or SUV.
The Scion Xb, which is not available with side airbags to protect vehicle occupants when rammed from the side, was given that lowest safety ranking.
The IIHS said the boxy Scion, designed and manufactured in Japan, had a side structure that allowed the simulated SUV that struck the car to penetrate the vehicle and strike the head of the crash dummy.
"Measures indicate the likelihood of brain injuries, serious neck injuries and a fractured pelvis in a real-word crash of similar severity," the IIHS report said.
The 2007 Toyota Yaris equipped without side airbags was also given a "poor" rating for side crashes.
Equipped with the side airbags, which represent an additional cost of $650 on the $12,000 base model, the Yaris earned a "good" rating in the side collision test.
Toyota said in a statement that the IIHS test procedures were "very severe," and noted that its cars meet federal safety requirements.
The Japanese automaker, which has seen its U.S. sales rise almost 13 percent this year, said it would provide standard side curtain air bags by 2009 on all of its vehicles.
The Accent from Hyundai Motor Co. and the Rio from its affiliate Kia Motors Corp. were given "poor" ratings in side and rear crash tests.
A Hyundai representative could not be immediately reached for comment.
Although all of the cars tested were rated "good" or "acceptable" in frontal crashes at 40 miles per hour, none of the cars except the Versa achieved a similar rating in a test intended to simulate a crash from the rear at 20 miles per hour.
Lund said that finding is significant because claims for neck and back sprains represent about half of the insurance industry's losses from automobile accidents.