The big American family sedan may be a gas-guzzler. But it can also be an insurance bargain, according to a new report by Allstate Insurance Co. on the costs of insuring different car models.

In contrast, luxury American and foreign cars, and sports models in general, tend to carry the most expensive premiums for collision and comprehensive coverages, the report said.

Collision-comprehensive premiums for small nonsports cars can also be expensive. But a small car that is relatively unattractive to thieves, and which tends to be bought by older people or married couples, might be eligible for lower premiums, according to the report by Allstate, the nation's second-largest property and casualty insurance company.

The annual report comes from the company's 9-year-old Make and Model Experience Program, which monitors the loss experience, from both damage and theft, of privately owned vehicles. State Farm Insurance, the country's largest property and casualty insurance organization, has a similar program, which produced similar findings.

The monitoring techniques and findings of both companies generally are used and shared by most of the nation's property insurers, according to June Bruce, spokeswoman for the New York-based Insurance Services Office. Bruce's organization collects statistics and makes nonbinding rate recommendations to the insurance industry.

The Allstate and State Farm programs, and similar efforts, "are very valid because they are based on the actual loss experience of specific makes and models," Bruce said.

The latest Allstate report focuses on 1984- and 1985-model cars. Cars with the least damage-theft experience had collision-comprehensive premium costs that were 25 percent to 55 percent lower than the average comparable coverage costs for other makes, the Allstate report said.

Only six models, five of them full-size American sedans, made the 25 percent-to-55 percent discount category in the last two model years. They included General Motor Corp.'s Chevrolet Impala and compact Oldsmobile Omega. Ironically, the 1980 editions of the Omega and other X cars are the targets of a $4 million federal lawsuit alleging that they were introduced with faulty rear brakes.

The remaining four cars in the group of models rated by Allstate as least costly to insure include Chrysler Corp.'s big Dodge Diplomat and Plymouth Gran Fury and Ford Motor Corp.'s Mercury Marquis and Mercury Grand Marquis.

The models with the highest insurance premiums, those with collision-comprehensive coverage costs 25 percent to 55 percent above average, included luxury models such as BMW cars, sports luxury models such as GM's Chevrolet Corvette, and Japanese sports cars such as the Mazda RX 7.

The intent of the Allstate program "is to identify those automobiles which have fared better or worse in terms of loss experience," said Susan R. Schneider, spokeswoman for Allstate.

"Since insurance premiums are affected, this information can be helpful to consumers who are shopping for new cars," she said.

Jean Crocker, GM's coordinator of insurance industry relations, said she agreed with Schneider's assessment, even though some GM cars fared poorly in the ratings.

The Allstate findings "are a good benchmark" for auto-loss experience, Crocker said. She said that Allstate and State Farm have been working with GM's designers and engineers since mid-1984 to improve GM vehicles' crash standards and theft resistance.