Ford Motor Co. on Wednesday said U.S. sales were down 3 percent in September, to nearly 181,000 vehicles, as the company transitions its production to the all-new F-Series aluminum-bodied pickup truck, due in showrooms by the start of next year.

As the automaker makes a historic transition to a lighter version of the longtime national-best-selling passenger car, it has managed inventories of the previous version, which pushed down deliveries by 1 percent.

That doesn’t explain away Ford's overall decline last month, followed by a flat August. Ford sedan sales are down about 6 percent, while truck sales, including the E-Series van that will be retired this year, fell 2.5 percent. The Ford Fusion was the only Ford sedan to grow sales, by nearly 9 percent, marking the best year-to-date sales performance for the mid-sized car since 2005.

“Fusion is selling strong, with five straight months of record performance and California driving more than 20 percent of our retail growth this year,” John Felice, a Ford vice president, said in a company announcement.

But the company’s other sedans experienced declines. Demand for the Focus compact, the second-best-selling Ford sedan after the Fusion, retreated 8 percent to 15,397, while Fiesta deliveries dropped 17 percent to 4,185. Mustang sales were negatively impacted as Ford scales down production of the current model and consumers wait for the 2015 version, due out this fall.

Utility vehicle sales also fell, by 2 percent, on flagging demand for the Escape and Edge.

The Lincoln luxury SUV group saw a 65 percent rise thanks to the Navigator and the MKX mid-sized crossover. Lincoln cars, however, didn’t fare as well. Demand for the MKZ and MKS sedans fell 28 percent.

Watch a recent Bloomberg News review of the upcoming 2015 Ford Mustang: