All eight models of small cars tested by the Insurance Institute for Highway Safety received passing scores in head-on crash tests, but only one, the Nissan Versa, received high marks in both side- and rear-crash tests. The Versa is several hundred pounds heavier than competing models and therefore was better able to withstand the test impact.

Three other models — Toyota Yaris, Honda Fit and Mini Cooper from BMW — scored well in side tests but received low ratings in rear tests.

The results, which are being released Tuesday, show that some small cars offer significantly better protection than others, but experts caution that even the safest subcompact car cannot overcome its inherent size and weight disadvantage. On average, subcompacts weigh about 800 pounds less than midsize cars like the Toyota Camry and 4,000 pounds less than midsize S.U.V.’s like the Ford Explorer.

“A good-scoring small and lightweight car is not nearly as good as a good-scoring midsize car — that’s just the law of physics,” the insurance institute’s president, Adrian Lund, said. “If you’re really shopping for safety, then this probably isn’t your best choice.”

Small cars can be forced backward more easily in crashes with larger vehicles, and their crumple zones are less able to protect the passenger compartment.

As a result, fatality rates for drivers in multiple-vehicle crashes are higher for subcompacts than for every other vehicle category — 83 deaths per million registered vehicles, more than double the average for all sizes of cars and trucks.
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The size of subcompacts means that without the buffer a side air bag creates, the front end of a truck or S.U.V. could strike an occupant’s head. But dealers say their customers rarely are willing to pay more for side air bags or wait longer for a vehicle equipped with them.