Americans took on $6.31 billion more in installment debt than they paid off in August, only slightly more than the July level, the government reported yesterday. The Federal Reserve Board said the August increase was only $65 million higher than the $6.25 billion in debt taken out in July. Total consumer debt outstanding was put at $512.08 billion at the end of August on a seasonally adjusted basis.

Some analysts had predicted a bigger increase for August based on the surge in auto sales that occurred during the month as buyers responded to attractive cut-rate financing deals being offered by auto makers. However, the increase in auto loans for August was only $2.36 billion, down slightly from the July level of $2.69 billion.

James Christian, chief economist for the U.S. League of Savings Institutions, noted that the Fed figures are adjusted to take into account normal seasonal swings. He said this adjustment leveled the pickup in auto sales since they normally rise in August.

The annual rate of growth in consumer debt in August was 15 percent, down from the 19 percent annual pace set from April through June. Christian said this moderation was good for the economy because it brought the debt increases more in line with gains in personal income.