ARLINGTON, Va. — US for-hire truck tonnage spiked 1.4% in September, matching August’s gain.

The American Trucking Associations (ATA) reported that the seasonally adjusted truck tonnage index is up 8.4% compared to September 2012, marking the largest year-over-year gain since December 2011.

Year-to-date, tonnage is up 5.4% over 2012.

“I continue to be pleasantly surprised on the strength of truck tonnage,” ATA chief economist Bob Costello said. “I attribute a part of tonnage’s robustness to the sectors of the economy that are growing fastest, like housing construction, auto production, and energy output. These industries produce heavier than average freight, which leads to faster growth in tonnage versus a load or shipment measure.”

Costello added that tonnage levels suggest the economy may be stronger than many believe it to be.

“While tonnage is likely running ahead of overall economic growth, perhaps the economy is stronger than many believe. The index has now increased in four of the last five months and the year-over-year growth rate has accelerated. Plus, other measures of truck freight volumes, while increasing at a slower pace than tonnage, have also accelerated in recent months,” he said. “However, the government shutdown served as a headwind in the fourth quarter.”