The American Trucking Associations' tonnage index dropped fell in August compared to September but showed modest growth over last year.

The ATA's advanced seasonally adjusted For-Hire Truck Tonnage Index contracted 0.9% in August after increasing 0.4% in July. However, July's gain was better than the preliminary no change ATA reported on Aug. 21. In August, the SA index equaled 118.3 (2000=100).

The month-to-month drop in August, while not erasing the cumulative 1.5% gain in June and July, was significant, notes ATA. Compared with August 2011, the SA index was 3.2% higher. Year-to-date, compared with the same period last year, tonnage was up 3.7%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 126.8 in August, which was 5.7% above the previous month.

"While there has been acceleration in housing during the last few months, truck tonnage is being weighed down by a flattening in manufacturing output and an unintentional increase in inventories throughout the supply chain," ATA Chief Economist Bob Costello said. "While choppy, tonnage has essentially been flat this year with August being the second lowest month of the year."

Costello also noted that the SA index in August was 0.3% below January 2012 and 1.4% less than the high in March.

"Expect tough year-over-year comparisons to continue through the rest of the year as tonnage grew nicely during the last five months of 2011," he said, adding the economy isn't expected to grow much in the second half of the year as manufacturing decelerates and excess inventories are worked off. As a result, tonnage is expected to increase less than 3.5% in 2012.

Analysts from Jefferies & Co. in an investor e-mail note that "Our channel checks suggest that loads (rather than tonnage) so far in September have ramped seasonally and turned positive on a YoY basis." However, they note, weekly diesel consumption continues to trail year-earlier levels in 3Q.