Trucks moved more freight in August, several industry measures showed, though not all fleets took part in the rebound.

The American Trucking Associations’ seasonally-adjusted truck tonnage index rose 5.7% in August from a month earlier to 141.8, just below a record set in February. The report follows DAT’s estimate last week of an 11% year-on-year increase in North American truck volumes in August.

The readings are a sign that freight demand may have bottomed out over the summer, after a months-long slump driven by high retail inventories and a weak manufacturing sector. While manufacturing activity remains uneven, inventories have begun to draw down due to lower import volumes and strong consumer spending.



“With moderate economic growth forecasted, truck freight will improve as progress is made with the inventory overhang,”said Bob Costello, the ATA’s chief economist.

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Still, Mr. Costello said one positive month isn’t enough for trucking companies to assume the worst is over. He said he expects the freight business to remain volatile until excess inventories work through North American retail and manufacturing supply chains.

Data from Cass Information Systems, which measures both truck and rail activity, showed U.S. freight shipments in August rose only 0.4% from the prior month, but declined 1.1% from August 2015. Cass analysts also highlighted increased volatility amid persistent weakness in volume and pricing. Parcel and expedited shipping for e-commerce has grown, according to Cass, as freight transportation serving manufacturing and construction industries remains soft.

Last month’s strong volumes come after several large trucking firms had given bleak assessments for August and September. At a recent meeting with analysts, John Steele, chief financial officer at Werner Enterprises Inc., described recent volumes as “relatively stable” though “not yet what we would classify as strong.”

Werner has pulled back on capital spending and reduced its fleet size in the third quarter.

Forward Air Inc. reported a decline in less-than-truckload shipments in August, and in recent comments on the company’s performance Chief Executive Bruce Campbell said unadjusted year-over-year daily tonnage was down 4.6% in the third quarter through Sept. 18.

Old Dominion Freight Line Inc., one of the largest less-than-truckload carriers in the U.S., reported a 1.4% decline in daily tonnage in August from the same month a year ago.

Corrections & Amplifications

Forward Air Inc. reported a 4.6% year-over-year decline in shipments in the third quarter while Old Dominion Freight Line reported a 1.4% decline in August tonnage. An earlier version of this article incorrectly stated that Old Dominion reported a 4.6% decline.

Write to Erica E. Phillips at erica.phillips@wsj.com