U.S. automobile manufacturers reported yesterday that car sales in early September tumbled 15 percent from year-earlier figures, ending a record-breaking late summer selling spree.

Separately, a New York research organization reported that consumers' expectations for the future of the economy brightened a bit in August but remained far below the optimism expressed at this time in 1977.

The big four auto firms reported combined sales totaled 134,642 units in the first 10 days of the month, down sharply from the 180,725 units sold a year ago. The daily rate of sales average 19,234 for the period, compared with a 22,591 daily rate last year when there was one additional selling day.

General Motors Corp., which led the industry to a record period 10 days earlier, led the decline this time with a 24 percent sales drop from year-ago figures. GM's sales totaled 65,735 units, compared with 98,806 in the comparable period last year. Company officials said the drop was "not unexpected" after GM's record August performance which was boosted by dealer incentive contests.

The No. 1 auto firm saw tis share of the domestic market drop from 62 percent in the closing August period to 48.8 percent in early September.

Ford Motor Co., though gaining only slightly from its year-ago performance, boosted its market share to 36.6 percent from the 24.2 percent share 10 days earlier. Ford's sales totaled 49,290 units for the period, up 0.7 percent from last year.

Chrysler Corp. reported a 19 percent drop in sales of 16,058 passenger cars. American Motors Corp. posted a modest 2.2 percent gain with sales totaling 3,559. Ford, GM and AMC's figures included early sales of 1979 models. Chrysler spokesman said the No. 3 auto firm will not begin reporting its '79 sales until official introduction later this month.

So far this year the big four autp firms have sold 6,478,946 passenger cars, up 2.1 percent from the 1977 period.

The report on lagging consumer confidence was published by private business research organization.

A monthly "consumer confidence index," based on interviews with 5,000 families nationwide, rose to 91.5 in August after dipping 3.7 points in July to 88.8. In February, when severe winter weather sharply restricted economic activity, the index was at its 1978 high of 103.8.

The index combines answers to questions on current business and employment conditions, on prospects for the next six months and on personal finances. The Conference Board has set 1969-1970 as the base period for the index, with a reading then of 100.

According to the survey, consumers believe the current economic situation has turned down slightly, measuring 91.7 in August against 92.5 in July. But their assessment of the future is more rosy, rising to 91.4 from 86.3 a month before.

More respondents told the Conference Board's poll-takers in August than in July that they intend to buy autos, homes, major applances and carpets in the next six months. But Federal Reserve figures suggest the nation's buying plans may be turning down, rather than up.