Sales of new U.S. cars in mid-January rose 4.1 percent from a year earlier, producing the highest sales rate for that period in 12 years, the six major domestic auto makers said yesterday.

Despite the moderate increase, the sales rate was considered strong given the poor weather in much of the nation in the Jan. 11-20 period, and given the robust year-earlier performance used for comparison.

In addition, the latest sales pace worked out to an annual rate of 8.4 million units; the auto makers actually sold 7.9 million vehicles in 1984.

The companies said they sold 168,668 domestic cars in the latest period, against 182,273 a year earlier.

The increase of 4.1 percent is based on the rate of sales per day, because there were eight actual selling days in the period this year, and nine selling days a year ago. The daily sales rate rose to 21,084 from 20,253.

It was the best mid-January showing since 1973, when the daily sales rate was 23,390 cars.

In the latest period, General Motors Corp. sales rose 6.6 percent from a year earlier, Ford Motor Co. sales slipped 2.4 percent and Chrysler Corp. sales gained 13.2 percent.

Among the smaller producers, American Honda Motor Co. Inc. said sales of its U.S. cars rose 16.2 percent from a year ago.

American Motors Corp. and Volkswagen of America Inc., however, extended their year-to-year declines, with AMC posting a 16.3 percent drop and VW a 34.1 percent decline.