American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index fell 4.3% in June, following a 6.9% gain during May. In June, the index equaled 138.5 (2000=100), down from 144.7 in May.

Compared with June 2016, the SA index increased 1.3%. In May, the index jumped 5.2% on a year-over-year basis. Year-to-date, compared with the first half of 2016, the index is up 1%.

As part of this report, ATA also revised its May gain in the index upward to a 6.9% jump from a previously reported 6.5% gain.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 144.3 in June, which was 1% below the previous month (145.8).

"After such a large spike in May, it was not surprising to see the index give back some of those gains in June," said ATA Chief Economist Bob Costello. "However, looking back at the second quarter as a whole, tonnage was up 0.8% over the first quarter and 1.9% over the same quarter last year, so it was a solid three month period.

"June's slide does not change my belief that we will continue to see moderate, albeit at times choppy, growth in truck tonnage as the year continues," he said.

Trucking serves as a barometer of the U.S. economy, representing 70.1% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled nearly 10.5 billion tons of freight in 2015. Motor carriers collected $726.4 billion, or 81.2% of total revenue earned by all transport modes.