All three factors were key to September's 10 percent sales increase. A surge in retail sales pushed the seasonally adjusted annual selling rate to 13.1 million light vehicles, the highest in five months.

"There is a lot of pent-up demand," said General Motors sales chief Don Johnson.

"Consumers are being cautious, but they are not out of the market. We think that will continue the rest of the year."

Auto executives say dramatic changes have occurred in the four months since vehicle shortages and troubling economic indicators derailed a budding auto recovery.

"Things are already changing -- and in a very big way," Toyota brand boss Bob Carter said last week. With resumed full production, larger dealership stocks and five new models this autumn, he pledged that September's 18 percent decline would be the last year-over-year loss this year for Toyota Motor Sales U.S.A.

"We expect to exceed year-ago sales levels beginning in October -- and we will continue to do so every month through the fourth quarter and beyond," he said.

The economic indicators normally linked with auto sales growth -- the unemployment rate, new-home starts, consumer confidence and financial market indices -- remain at or near several-year lows.

And analyst Itay Michaeli of Citi Financial warns that the number of vehicles per licensed driver may continue to decline since peaking at 1.18 in 2007, further dampening demand.

Despite the considerable headwinds, automakers still see improvement ahead.

For example, Jonathan Browning, CEO of Volkswagen Group of America, expects U.S. auto sales to end the year at 12.85 million, then rise to 13.5 million to 14 million in 2012.

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