The trade association also revised downward its June index from an increase of 2.8 to 2.6 percent.
Tonnage has fallen in three of the last four months on a sequential basis.
The index is adjusted to account for seasonal fluctuations in shipping volumes.
Compared with July 2010 tonnage grew 3.9 percent while June registered a 6.5 percent increase.
'Despite a solid June, our truck tonnage index fits with an economy that is growing very slowly,' ATA Chief Economist Bob Costello noted. 'The good news is that tonnage continues to increase on a year-over-year basis, but it is likely that the rate of growth will moderate in the second half of the year.'
Most economists are now predicting slower growth in the second half than previously expected and a peak shipping season that is less robust than normal.
