October U.S. auto sales should be down about 6 percent from a year ago, marking the first single-digit monthly decline since May 2008, influential industry forecasting firm J.D. Power and Associates predicted on Friday.

J.D. Power forecast October sales at 10.3 million units on an annualized basis, compared with 10.5 million units a year earlier.

While year-over-year comparisons benefit from a low selling base in October 2008, improvements in consumer confidence and credit are propelling the return to positive sales gains relative to last year, said Gary Dilts, senior vice president of global automotive operations at J.D. Power and Associates.

While the forecast sees the first single-digit drop in sales in 17 months, August 2009 actually showed a 13 percent increase in annualized sales from a year before. However, that month was seen as an anomaly because of higher sales generated by the federal Cash for Clunkers incentive program.

(Reporting by Soyoung Kim and Bernie Woodall, editing by Gerald E. McCormick)