A one-time surge in farm-subsidy payments helped drive personal incomes up 1.7 percent in October, and Americans saved the extra wealth instead of spending it, the government reported yesterday.

The 1.7 percent figure matched the monthly advance in July 1981, which was the largest since a 2.5 percent gain in June 1975.

Consumer spending in October was flat as auto sales fell for the second consecutive month.

In other economic developments:

The Commerce Department reported a 0.3 percent rise in orders to U.S. factories for durable goods, a moderate advance propelled by a sharp jump in aircraft orders. It followed a 2.4 percent surge in orders in September. Analysts said the gains showed rising overseas demand for U.S. products.

The National Association of Realtors reported that sales of existing homes climbed 3.2 percent in October, the best showing since May, despite the fact that mortgage rates were rising sharply in the first half of the month and the stock market fell in the last part of October.

The rise in personal incomes occurred mainly because farm incomes shot up 90.7 percent to an annual rate of $71.7 billion in October because of a big jump in government subsidy payments.

Analysts attributed this increase to a decision by the government to withhold subsidy payments in September in order to reduce the federal budget deficit for 1987 and push those payments into October, the start of a new budget year.

The overall income increase was 1.7 percent. Without the jump in farm-subsidy payments, the income advance would have been 0.7 percent, very similar to the 0.6 percent overall income gain recorded in September.

In Santa Barbara, Calif., White House spokesman Marlin Fitzwater said the durable goods report showed that "the future remains very bright in terms of manufacturing production." He called the personal income report "good news," and said there is reason for optimism "on both fronts." President Reagan is in California for Thanksgiving.

Analysts said the various reports present a picture of an economy with surprising strength in early October. But they said it remained to be seen how much damage was done by the record 508-point decline in stock prices on Oct. 19.

"All of these numbers are October numbers, and it isn't clear how important they are in the post-crash world," said David Berson, senior economist at the Federal National Mortgage Association. "But it does show that things were moving very strongly before the crash."

Of particular interest was the rise in home sales, which pushed them to a seasonally adjusted annual rate of 3.56 million units.

"The higher interest rates should have throttled sales in the first half of the month, and then the stock market crash added enormous uncertainty at the end of the month, but sales still went up," said Warren Lasko, executive director of the Mortgage Bankers Association.

A big question is whether consumers will continue to spend and keep the country out of a recession, or whether they will become fearful about the future and start building up savings.

While consumer spending was flat in October and savings rose sharply, analysts said this was the result of special factors other than fears generated by the stock market collapse.

"It is too early to tell whether consumers have put away their checkbooks," said Cynthia Latta, an economist with Data Resources Inc. of Lexington, Mass. "We will have to see the November data before we can begin to determine the impact of the market decline."

Consumer spending showed no improvement in October, holding steady at an annual rate of $3.01 trillion after declining 0.3 percent in September. The weakness in both months came from declines in auto sales following an August sales boom spurred by cut-rate financing deals.

Spending on nondurable goods, items expected to last less than three years, posted a good gain, and spending on services showed the biggest increase of all, a gain of $18.6 billion, reflecting in part payment of brokers' fees on the record level of stock sales, analysts said.

Disposable, or after-tax incomes, were up 1.8 percent in October, the biggest advance since a 3.2 percent one in May.

The combination of rising incomes and flat consumer spending left the savings rate -- savings as a percentage of disposable income -- at 4.7 percent in October, up sharply from a rate of 3 percent in September. The savings rate was the highest monthly figure since 4.9 percent in January.

The growth in incomes includes a $15.3 billion annual rate of increase in the important category of wages and salaries. This was slightly ahead of a $12.6 billion rise in wages in September.

The various changes left personal incomes at an annual rate of $3.84 trillion in October.