Truck tonnage handled by the nation's for-hire carriers in June rose 1.2 percent from May's results, the largest month-over-month gain so far this year, the American Trucking Associations (ATA), the trade association representing large motor carriers, said today.

Still, the June numbers—which are seasonally adjusted—did not do much to offset the weakness of the prior two months, or change the group's view on tonnage for the balance of the year. The index dropped a cumulative 2.1 percent in April and May, and May's decline was revised from initial projections to show a larger drop-off than originally reported.

Bob Costello, ATA's chief economist, said today he has lowered his outlook for 2012 tonnage growth to 3 percent from 3.5 percent, citing the oft-noted economic uncertainties both in the United States and overseas.

"June's increase was a pleasant surprise, but the lower year-over-year gain fits with an economy that has slowed," Costello said. "Manufacturing output was strong in June, which helped tonnage levels."

Year-over-year, the seasonally adjusted June tonnage index rose 3.2 percent. That is the smallest year-over-year gain since March, ATA said. Year-to-date, compared with the same period last year, tonnage was up 3.7 percent.

The not-seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 123.0 in June, which was 0.9 percent below the previous month.

According to ATA, trucking moves two-thirds of all U.S. tonnage handled by all transport modes. Trucks hauled 9.2 billion tons of freight in 2011, with truckers earning $603.9 billion revenue, or about 81 percent of revenue earned by all transport modes, the group said.