Americans saved the smallest part of their incomes ever last month as consumers went all out in purchasing automobiles at discount financing offered by auto companies, the Commerce Department reported yesterday.

Many consumers bought automobiles they would have purchased next year, or wouldn't have bought at all, except for the cut-rate financing offered by auto makers to clear surplus vehicles from their stocks, economists said.

The growth in spending far outpaced the increase in incomes. The Commerce Department reported that Americans' personal income rose a modest 0.3 percent in September, following an equal rise in August, while personal spending on goods and services increased 1.2 percent both last month and in August.

Income after taxes was up 0.2 percent both months.

During September, the saving rate -- the percentage of after-tax income saved -- fell to 1.9 percent, the lowest since the third quarter of 1950, when it was 2.6 percent. The rate had been 2.9 percent in August.

Economists said yesterday that the saving rate cannot stay that low for long, and eventually consumers will begin buying less and contributing to a slowdown in economic growth.

Saving is calculated by computing the difference between income and consumption. However, during August and September most of the surge in consumption on automobiles was done on credit rather than with cash, making it appear that consumers saved less than they actually did, economists said.

However, because of continued consumer spending, historically high consumer debt levels and the slow growth in income, economists said they expect consumer spending to slow in coming months, causing a dismal Christmas selling season.

The White House, in a statement, ignored the savings figures, instead praising the increase in personal income and spending.

"Like virtually all of the recent economic news, today's figures show an economy that continues to grow at a healthy pace," said White House spokesman Larry Speakes. "The personal-income figures are especially welcome for they reflect the pocketbook benefit for all Americans that is the result of this administration's stewardship of the economy."

Private economists were less sanguine, emphasizing that the saving rate is so low that soon consumers will begin saving more and buying less.

"This has some pretty disturbing implications for economic growth up ahead," said Edward Yardeni, chief economist for Prudential-Bache. "Many economists had expected that the consumer would keep the economy growing. With this sort of low saving rate . . . there's no purchasing power left."

Yardeni said the decline last month in savings was not an aberration. "The trend has been pretty clear toward a lower saving rate," he said. "A lot of consumers went out and bought cars they really didn't need today or that they really couldn't afford. They're not going to have much to spend at Christmas time."

Commerce Department chief economist Robert Ortner said the low savings rate means "we can't count on the consumers to keep pulling the economy up. What we're going to see in the fourth quarter is a substantial shift in the composition of economic activity from the consumer sector toward rebuilding of inventories," increased purchases by businesses and the defense industry and improved home-building activity "while the consumers reload their ammunition."

"The significance is that consumers by increasing their spending have cut their savings to historically low levels, raised their debt to historically high levels and I think this won't continue," Ortner said. "It would be serious if we had a slowdown in consumer spending which persisted for a protracted period of time, because consumers make up two-thirds of the economy."

John Hammond, an economist for Data Resources Inc., said consumers continue to spend because they are confident about the future. Consumers are "pretty comfortable about the way things are right now and they don't have to save for contingencies such as unemployment or other adversities," Hammond said.

The saving rate has dropped from 5.0 percent in 1983 and 6.1 percent in 1984 to an average of 4.0 for the past six months. Commerce reported that the personal consumption deflator, an inflation measure, increased 0.1 percent in August following a 0.1 percent rise in July. Private wages and salaries in September increased 0.8 percent following a 0.6 percent increase in August.