Domestic automakers yesterday reported a 6.2 percent sales increase in early May, moving the industry's modest sales recovery into its third month.

Chrysler Corp. and American Motors Corp., both of which had weak early-May sales last year, posted the greatest gains. AMC, still paced by sales of the Renault Alliance, sold 5,100 cars, up 104 percent over early May 1982. Chrysler showed a 41.6 percent increase, selling 22,737 new cars in the first 10 days of the month compared with 16,052 sold last year.

General Motors Corp. sales dropped 1.6 percent in the early part of the month, down to 102,302 from 103,977 in the year-ago period. GM officials said they had their strongest early-May sales in recent years in 1982, and that their slight drop this year simply reflected the difficulty in matching last year's gains.

But GM's Cadillac division had no need for excuses. Sales of the big luxury cars jumped 20.8 percent over last year's figures, to 7,897 from 6,535 in the year-ago period.

Cadillac's 1983 year-to-date sales of 98,772 new cars were up 22 1/2 percent from the 80,625 Cadillacs sold in the same period in 1982.

However, GM Chairman Roger Smith said in a recent interview that the increase in big-car sales does not necessarily represent consumer abandonment of smaller, fuel-efficient models.

"We don't have that many people moving from Chevettes to Cadillacs. What we have are former Cadillac owners trading in for new Cadillacs," he said.

Ford Motor Co., the second-largest domestic automaker after GM, reported selling 35,574 new cars in the first 10 days of May (which contained 8 selling days), up 1 1/2 percent over last year's period. Again, difficulty in matching last year's relatively strong selling period was blamed for this year's performance.

But Ford officials, buoyed by strong sales of their new Thunderbird and Cougar lines and optimistic about future sales of their soon-to-be-introduced Ford Tempo and Lincoln/Mercury Topaz models, expressed confidence that their sales volume will increase as the year progresses.

The optimism from Ford's Dearborn, Mich., headquarters is backed with plans to increase the company's new-car production in the third quarter by 49 percent, up to 321,000 units from 215,000 in the third quarter of 1982.

Volkswagen of America, which had been posting consecutive losses, showed a 10.8 percent increase in new-car sales in early May to 1,811 cars from 1,635 sold last year.

Overall, companies making cars in the United States, including Honda in Marysville, Ohio, sold 168,785, a 6.2 percent increase over 158,903 sold in the first part of May last year.

Honda, which began producing cars in the United States this year, sold 1,261 cars in early May.