With gasoline prices rising and increased talk about global warming, little fuel-efficient cars like the Smart Fortwo, Toyota Yaris or Honda Fit may seem attractive. But are they safe?

Knowing for sure is an enormously complex task. One problem is human ingenuity. People find so many ways to crash that one can't test for all of them. Also, cars of different sizes attract drivers of different ages and different skill levels.

Some side-impact crash tests done by the Insurance Institute for Highway Safety have indicated there can be exceptions to the bigger-is-better rule, but fatality statistics show that, overall, small cars are more dangerous. Occupants of the smallest cars are 51 percent more likely to be killed in a crash than those in mid-size or large cars, according to 2006 fatality figures.

Over the last 10 or 15 years, small cars have become more crash-worthy, said Adrian Lund, president of the institute. But larger vehicles have also gotten safer.

"So, the fact is that size and weight ... are still an important factor," Lund said.

Put another way, having better fuel economy and being surrounded by air bags does not counter the unyielding freight train of physics. Consider the differences in weights.

Mini-cars typically weigh 2,500 pounds or less. The Smart Fortwo weighs about 1,800 pounds. A mid-size sedan is about 3,300 pounds. A large sedan is about 4,000 pounds, and a four-wheel-drive sport utility vehicle is about 5,500 pounds.

"Almost everything you hit in the real world is more capable of causing damage to you if you are in a small, lightweight car than in a larger, heavier one," Lund said.

Small vehicles are among those crash tested by the National Highway Traffic Safety Administration and the insurance institute. In many of those tests, small vehicles get top ratings. But there is some crucial fine print: In frontal crash tests the vehicles can be compared only against other vehicles of similar size and weight. That's because in a frontal crash test the vehicle hitting a barrier provides the amount of striking force.

So, while the 2008 Honda Fit, Toyota Yaris and Mini Cooper got "good" ratings, the highest, from the institute, that means they are only safer than another small car with a lower rating. They are not necessarily safer than a larger car with a lower rating. Side-impact crash tests are different. The impact comes from a ram that strikes the car. Because the striking force is the same for each test it is possible to compare vehicles of different sizes.

That can result in some surprises when comparing a well-engineered small car and a large car that is an old design or poorly engineered. The Honda Fit, for example, got a "good" rating for side-impact protection from the insurance institute. The 2008 Ford Crown Victoria got a "marginal" rating. "Marginal" is one step better than the institute's worst rating of "poor."

"If you are in the crash that we did for the Fit and the Crown Vic then you are better protected in the Fit than in the Crown Vic. Period," Lund said.

Crash ratings are available at www.iihs.org and www.safercar.gov.