CHICAGO (Reuters) - U.S. heavy-duty truck orders fell 27 percent last month versus the same period a year ago, marking the worst September since 2009 amid ongoing uncertainty over the economy among truck firms, according to preliminary data from industry forecaster FTR.

"Fleets are cautious due to an uncertain economy and slow freight growth," Don Ake, vice president of commercial vehicles at FTR, said in a statement. "Class 8 (truck) inventories also remain high and this also restrains new orders."

FTR said orders for large trucks hit 13,800 units in September, a number not reached for that month since the height of the Great Recession.

Class 8 trucks are the workhorse of America's economy, hauling around 70 percent of the country's freight.

Sales have been weak this year amid lackluster retail sales and industrial output. Industry officials and analysts predict sales will start to rebound in 2017 as long as the U.S. economy grows at a more stable rate.

"Large fleets are expected to begin ordering replacement units for 2017," Ake said. "If the economy does improve and the trucking outlook brightens, then medium-sized fleets and others should feel confident enough to order also in coming months."

(Reporting by Nick Carey; Editing by Matthew Lewis)