The American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index increased 1.4% in August after falling 0.6% in July. 

The latest gain was the largest since May. In August, the SA index equaled 126.9 versus 125.2 in July. Compared with August 2012, the SA index surged 6.9%, which is the largest year-over-year gain since December 2011.

Year-to-date, compared with the same period in 2012, the tonnage index is up 5%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 131.3 in August, which was 1.5% above the previous month (129.4).

"The strength in tonnage continued again in August, with the index increasing in three of the last four months," ATA Chief Economist Bob Costello said. "The improvement corresponds with a solid gain in manufacturing output during August reported by the Federal Reserve last week."

"However, tonnage's strength in recent months, and really through 2013, is probably overstating the robustness of the economy and trucking generally," Costello said. "It just so happens that the sectors of the economy that are growing the fastest - in housing starts, auto production, and energy output, primarily through hydraulic fracturing - produce heavier than average freight, leading to accelerated growth in tonnage relative to shipments or loads.

"Truckload industry loads have accelerated the last few months, but are flat for the year, while less-than-truckload shipments are up less than 1.5% in 2013," Costello added.