Americans took out $6.06 billion in credit in September for the biggest increase in a year, the Federal Reserve reported. The Fed said consumer debt rose at an annual rate of 12.2% in September and was the largest advance since a $7.6-billion rise in credit in September, 1986. Both months were heavily influenced by car sales spurred by dealer incentive programs. Consumer credit rose $5 billion in August. Auto loans climbed at an annual rate of 15.3% in September, rising by $3.24 billion following a $3.03-billion August gain. Consumers also borrowed heavily on their credit cards, boosting this category by $2.02 billion in September, almost double the $1.09-billion August increase. Credit card debt rose at an annual rate of 17.4%, the largest advance since April, 1986.