ARLINGTON, Va. — US for-hire truck tonnage spiked 1.4% in August, on the heels of a 0.6% decline in July.

However, the American Trucking Associations’ cautioned the impressive tonnage is attributed to the type of freight being hauled, and not necessarily increasing demand.

The ATA index posted its highest month-over-month gain since May. It was 6.9% higher than August 2012, which is the largest year-over-year gain since December 2011.

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

“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 explained: “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.”