But the surge has not been sustained. In September, the nation’s savings rate dropped for the third consecutive month, the Commerce Department said Friday. It is now at 3.6 percent of personal disposable income, its lowest level since the month the recession began.

The latest decline raises the question of whether consumers are returning to their old spendthrift habits or were temporarily relaxing budget restrictions to make long-awaited purchases.

Real personal after-tax income declined in September, just as it did in July and August. Even so, consumers spent more, for an increase of 0.6 percent in September.

The increase in consumer spending was widely embraced as good news, a sign that consumers might be helping to propel the economy forward. Consumers account for roughly two-thirds of economic activity.

Economists warn, though, that increases in spending will be hard to sustain if Americans are doing it by simply putting a little less away every month.

Some of the spending increase was to cover necessities like medical bills and gasoline. Consumers also spent more on furniture and goods like televisions.

Scott Hoyt, an economist at Moody’s Analytics who specializes in consumer spending, said there were two competing hypotheses as to why the savings rate had dropped. “One is that consumers have just decided that they need to spend — they need to replace the car, the appliance, they want a new wardrobe.” The other, he said, is that the data, which is often revised months down the road, is simply incorrect.

“There have been several times where we spent a year or more talking about a negative savings rate” — meaning consumers spent more than they took in — “only to get benchmark revisions to the data,” Mr. Hoyt said. “The savings rate’s never been negative.”

Mr. Hoyt said he leaned toward the second explanation, in part because more spending was less likely with credit tight and consumer confidence at recession-level lows. But, with appliances and cars aging, there is pent-up demand for replacements.

In the decade leading up to the recession, Americans socked away an average of only 3.1 percent of their income, a much lower rate than in Europe, China, India and Japan.

After the crisis began, Americans raised their savings rate to above 5 percent. It slipped briefly below 4 percent in mid-2009 before climbing again.

Paul Ashworth, chief United States economist at Capital Economics, offered a partial explanation for the latest change. With interest rates so low, people who have money are earning lower returns, while people who owe money can service their debts for less. In effect, this has resulted in a transfer of wealth from people who are more likely to save — say, someone nearing retirement age — to someone more likely to spend — say, a young couple with a mortgage and a car loan.

But that would not account for the whole shift from saving to spending, he said. “Maybe households were desperate and didn’t know what else to do,” he said. “You can put off some discretionary spending, but there is a level of spending that you have to follow through on.”

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