By MARTIN CRUTSINGERWASHINGTON (AP) â¿¿ Americans stepped up borrowing in December to buy cars and attend school. But they cut back sharply on credit card use, continuing a trend that could hold back growth this year. Consumer borrowing rose $14.6 billion in December from November to a total of $2.78 trillion, the Federal Reserve said Thursday. That's the highest level on record. The increase was driven entirely by gains in student and auto loans. Borrowing in the category that measures those loans increased $18.2 billion to $1.93 trillion. That's the biggest monthly gain since November 2001. Credit card debt, however, fell $3.6 billion to total roughly $850 billion. Total credit card debt has declined 17 percent since July 2008. Americans have been relying less on their credit cards since the Great Recession. And December's decline could also be a signal that consumers were worried about higher Social Security taxes that began lowering take-home pay this year. "High unemployment and the increase in payroll taxes will leave households reluctant to run up big credit card balances," said Paul Edelstein, director of financial economics at Global Insight. "An unwillingness to take on this form of debt will limit the speed at which consumer spending will grow this year." The Federal Reserve's monthly consumer credit report does not separate auto loans from student loans. But according to quarterly data compiled by the Federal Reserve Bank of New York, student loan debt has been the biggest driver of borrowing since the recession. Student loans have risen 63 percent since mid-2008 to total $956 billion as of September of 2012, according to the New York Fed. The increase partly reflects high unemployment, which has led many Americans to seek better education and skills in a more competitive labor market. Auto loans are slightly lower than their total level in mid-2008. But they have risen roughly 9.4 percent since mid-2010 to total $750 billion as of September, according to the New York Fed report.