Now automakers and parts supply companies are starting to worry about being able to make enough cars and components, said Schuster. Many suppliers cut their operations or went out of business altogether during the recession, and are having a hard time ramping back up to meet demand.

Last year U.S. unemployment eased, the housing industry started to recover and people felt a bit more confident in the economy. Interest rates also stayed low and banks made loans available to more customers, even those with lousy credit. People began to replace cars and trucks that they'd owned since before the economy went sour in 2007. The average age of a vehicle in the U.S. grew to a record 11.2 years.

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