A report issued yesterday by an auto insurance industry group says that small Japanese cars are significantly more dangerous than American-made cars of comparable size, based on a study of government data on fatal auto crashes.

The Insurance Institute for Highway Safety, a research group supported by U.S. auto insurance firms, said its study is the first comparison of highway fatalities involving U.S. and Japanese-built cars.

The study compares the number of fatalities of drivers and occupants by car model with the numbers of registered cars of each model to obtain a fatality ratio. The study divides subcompacts into two categories, calling those with wheelbases of 96 inches or less small subcompacts.

In the case of single-vehicle crashes in 1980, there were 16.4 fatalities for every 100,000 Japanese-made small subcompacts registered in the United States, compared with 11.6 fatalities for 100,000 U.S.-built cars of comparable size.

The institute also compared fatalities resulting from collisions between small cars and other cars, showing that there were 23.6 fatalities per 100,000 Japanese-built cars and 18 fatalities for the American-built cars.

One of the nation's major insurance companies, the United Services Automobile Association, held a series of news conferences yesterday to call attention to the study. The association is a cooperative group for military officers.

"The safety advantages of American-made cars should be made known," said Robert F. McDermott, president of the association. "We are not trying to persuade policyholders to drive particular cars, only providing information to consider in reaching their buying decisions."

Neither the institute nor the USAA offered theories to explain the reported difference between the U.S. and Japanese-built cars. Although Japanese small subcompacts have a slightly smaller wheelbase than comparable U.S.-made cars, there is no such distinction for compacts.

The data on fatalities by car model are collected by the National Highway Traffic Safety Administration through its Fatal Accident Reporting System. The NHTSA had reported previously that occupants of small cars are in much greater risk of death or injury in collision with large cars because of the difference in impact forces related to vehicle weight.

The new study supports that conclusion. In all crashes reported in 1980 there were 15.6 deaths per 100,000 registered full-sized cars, 23.8 fatalities involving compact and small compact cars and 34.1 fatalities involving small subcompacts.

Looking at crashes involving only a car, the study reported 7.2 fatalities per 100,000 large cars, 11.4 fatalities for compact and small compact autos and 14.4 fatalities for small subcompacts.

"The occupants of small subcompact cars are more than twice as likely as people in full-size cars to die in single-vehicle crashes," the institute reported. "In small subcompacts, there are almost twice as many single-vehicle frontal crashes with occupant fatalities per registered vehicle as in full-size cars, nearly four times as many fatal single-vehicle rollover crashes, and more than four times as many fatal single-vehicle crashes involving occupant ejection."

One possible explanation is that roadside barriers and other safety structures designed for larger cars may increase the risk of fatality in small-car accidents, the institute suggested.

"For instance, concrete barriers designed to guide straying vehicles weighing 4,000 pounds or more safely back on to the road may cause smaller ones to flip over. Similarly, posts and lamps designed to break away in crashes may not perform as intended when struck by lower, lighter cars."

The institute also examined crash data in state police files in Maryland and North Carolina for 1974 through 1979. In Maryland, drivers of small subcompacts were almost 2 1/2 times as likely to be killed or injured as drivers of the largest passenger cars. In North Carolina, the margin was nearly 2 to 1.

The insurance institute urged all manufacturers to continue efforts to improve car safety. It noted the actions taken last year by Honda Motor Co. to correct safety defects in its 1980 Civic model revealed in the NHTSA's 35-mph-crash tests. After Honda strengthened and relocated safety belts and moved the air conditioner away from the steering column, the Civic was retested and passed by a wide margin.

"The trouble with many of today's small cars is that they are essentially scaled-down versions of large cars," said the USAA. "They are not designed and engineered with the special problems of smaller size, lighter weight and lower profile in mind."

It is possible to improve the crash-absorbing capability of small cars without significantly sacrificing energy efficiency, the association said. A layer of plastic could be attached on the inside surface of windshields to reduce lacerations in crashes. The NHTSA has been asked to permit the use of this kind of windshield in the United States.

Other safety features have been demonstrated on a research safety vehicle built for the NHTSA and designed to protect occupants in frontal crashes at up to 50 mph. The research car has foam-filled steel sections to absorb crash energy, flexible bumpers, hood and front fenders, and an advanced air-bag system. It achieves between 40 and 50 miles a gallon in highway driving.

"These improvements are feasible now, using present technology, and would mean the difference between life and death for many small-car occupants," the USAA said.