The American Trucking Associations' seasonally adjusted For-Hire Truck Tonnage Index fell 6.1 percent in May following a 7 percent surge in April, the group said Tuesday.

The Index increased slightly year-over-year, rising just 0.9 percent compared with last May, its smallest year-over-year gain since April 2017, according to ATA.

"As expected, tonnage corrected in May from the surprising surge in April," ATA Chief Economist Bob Costello said in a prepared statement. "The economy is still growing, but the recent volatility in truck tonnage fits with a broader economy that is showing more mixed signals. The good news is if you ignore recent highs and lows, tonnage appears to be leveling off, albeit at a high level."

The May index registered 114 compared with a revised figure of 121.4 in April, the association said.

ATA's For-Hire Truck Tonnage Index gauges tonnage carried by all modes of domestic freight transportation and is based on monthly member surveys.