DETROIT, JULY 6 -- Sales of U.S.-made cars fell sharply in late June, and economists for the nation's top three auto makers told trade publications they have lowered their sales forecasts for this year.

Overall car sales for all of 1987, including imports, are now expected to be about 10 million vehicles, about half a million cars fewer than previously anticipated and substantially below the 11.5 million cars sold in 1986, according to the economists and a group of industry analysts, whose views were reported by two trade publications.

They attributed the downgraded forecasts to a weaker-than-expected economy and the tax reform act, which caused a rush in car sales in late 1986 so buyers could take advantage of soon-to-expire tax breaks.

Economists for General Motors Corp., Ford Motor Co. and Chrysler Corp. all told Automotive News they expected some "payback" in 1987, but said they underestimated the extent of it.

Despite the lowered forecasts on car sales, the economists raised their estimates of light truck sales, now saying they will match or slightly surpass the record 4.7 million sold in 1986.

The outlook for 1988 is expected to be about even or up slightly from 1987 levels, Ward's Automotive Reports said. All nine industry analysts surveyed by the publication said they were optimistic for 1988 sales, saying they could be as high as 10.4 million units.

For domestically produced cars, 1988 could be another weak year at only 7 million units as imports continue to steal sales. U.S. car sales totaled just over 7.8 million units last year and nearly 8.2 million in 1985.

This contrasts with 9.2 million domestic car sales in 1977 and more than 9.3 million in 1965.

GM, Ford and Chrysler are internally forecasting between 10.1 million and 10.2 million car sales for 1988. They also believe the industry will support about 4.8 million light truck sales next year, the Ward's report said.

Analysts polled by Ward's agreed the vehicle market is now suffering because of strong 1986 sales but said they did not see any fundamental weakness in the nation's economy.

They noted that the current economic recovery will be 57 months old this August and that since 1954, only two expansions have lasted more than 58 months.

For the June 21-30 period, sales of U.S.-made cars fell 13.8 percent compared with robust sales in the same period a year ago, the domestic automakers reported today.

Light truck sales, however, continued to surpass last year's sales. Domestic makers sold 6.8 percent more light trucks in the eight selling days from June 21-30 this year than last year.

Domestic makers sold 229,407 cars and 134,170 light trucks during the period. Domestic car sales moved at a 7.3 million seasonally adjusted annual rate during the period and a 7.2 million annual rate during June.

Ford Motor Co., which had a very strong late-June period last year, sold 62,719 U.S.-made cars, down 9.7 percent from a year ago. Ford sold 45,664 light trucks, up 0.3 percent.

General Motors Corp.'s car sales fell 22 percent from last year, to 108,619 The industry leader's truck sales also fell, and were down 6.7 percent to 47,361.

Chrysler Corp.'s car sales fell 24.6 percent to 31,651. Chrysler truck sales, however, rose 50.9 percent, to 26,707.

American Motors Corp.'s car sales were off 31.5 percent, to 1,819 from a year ago, but its truck sales rose 43.4 percent, to 11,138.

West Germany's Volkswagen, which makes only cars in the United States, sold 2,514, or 7.4 percent fewer than a year ago.

Among Japanese automakers who build cars in this country, Honda's sales rose 1.1 percent, to 10,057.

Nissan's sales were an estimated 10,800 for the period, up from 50 a year ago, reflecting escalation of production.