American motorists would save about $2 billion in premiums if insurance companies took full account of the drop in accident claims due to a decrease in driving, a consumer organization declared yesterday.

Robert Hunter, founder of the National Insurance Consumer Organization, cited industry data showing that all types of auto insurance claims declined an average of 6 1/2 percent last year. Yet Hunter found in a sampling of 48 rate filings made by insurance in the East that they had requested premium increases on average of between 5 percent and 10 percent when the data supported no increase at all or a reduction of 5 percent. (All proposed rates contain provisions for inflation.) That works out to a $30-a-year average overbilling for the policy-holder, he said.

Hunter, a former federal insurance commissioner, called on insurance commissioners in each state to review rate filings to ascertain whether increases are justified.

The Insurance Information Institute, an industry trade organization, disputed Hunter's assertion, saying "the continuing impact of inflation on auto insurance claims is offsetting the reported reductions in traffic accidents attributed to reduced driving and the 55-mile-per-hour speed limit." The spokesman pointed out that the costs of auto insurance rose just 6.4 percent between January 1980 and the first of this year, or about half as quickly as the cost of living. During January 1981, the consumer price index showed the cost of auto insurance declined by one-10th of one percent.

At a news conference Hunter cited Department of Transportation estimates that motorists drove 4 percent fewer miles last year, largely as the result of a near doubling of gasoline prices in the past two years and of unemployment. The number of accidents decreased between 3 percent and 30 percent, depending on the state, while the number of nonfatal injuries dropped between 1 1/2 percent and 34 percent.

The industry said the increased cost of parts, labor and hospitalization offset reductions in the number of claims. For example, property damage liability insurance claims were down 12 percent, while average loss payment increased 12 percent. But, while bodily injury liability insurance claims were down 8 percent, the average loss rose 17 percent. Hunter countered that some companies whose rate filings he analyzed were using old data that did not reflect current decreases in claims.

Quoting industry trade publications, the institute said premium rates already have begun to fall in the West. It concluded, "If factors affecting insurance costs indicate that rates should be reduced at some point in the future, competition among insurance companies will assure that auto insurance premiums will reflect any savings in the costs of providing protections."