API said petroleum deliveries, a measure of demand, were at 18.2 million barrels per day in September, a 3.8 percent decline from the same period last year.
"The September demand numbers indicate there's still substantial weakness in the economy," API chief economist John Felmy said in a statement. "While manufacturing and employment have improved some, we've yet to see strong momentum developing."
The Organization of Petroleum Exporting Countries, in its market report for September, said the U.S. economy is expected to grow 2.0 percent next year, 0.3 percent less than this year.
Trade group API reported that overall demand for petroleum products was down 2.4 percent compared with last year.
Domestic crude oil production approached the 6.3 million bpd mark in September, the highest total for that month in 15 years. Crude oil imports declined 1.1 percent to average 8.8 million bpd in September.