In June, the American Trucking Associations' advance seasonally adjusted For-Hire Truck Tonnage Index fell 1.4 percent from May, an indication the economy's growth could be slowing.

While May's reduction was revised from 0.6 percent to 0.1 percent, May and June marked the first back-to-back contractions since March and April 2009.

However, the index was still up 7.6 percent from June 2009, the seventh straight year-over-year boost. Year-to-date, tonnage is up 6.6 percent from the same period in 2009.

The latest tonnage reading lowered the SA index from 110.1 (2000=100) in May to 108.5 in June.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, settled at 115.9 in June, up 6.5 percent from the previous month.

According to Bob Costello, chief economist of the ATA, the two sequential decreases reflect an economy that is slowing. He says growth in truck tonnage is likely to flatten out in the months ahead as the economy decelerates and year-over-year comparisons become more difficult. However, Costello also believes that tonnage doesn't have to grow very quickly at this point to make fleets happy, since industry capacity has been reduced by so much. "Due to supply tightness in the market, any tonnage growth feels significantly better for fleets than one might expect."