American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index fell for the first time since September, losing 5.7% in December. In December, the index equaled 142.9 (2000=100), down from 151.6 in November.

For all of 2017, compared with 2016, the index was up 3.7%. This was the largest annual gain since 2013 (6.1%).

Compared with December 2016, the SA index increased 5.9%, which was down from November's 7.5% year-over-year gain, but still very strong. In October, the index surged 10.5% on a year-over-year basis.

ATA also revised its November monthly increase in the index down to a 2.1% gain from the previously reported 2.3% increase.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 141.9 in December, which was 3.4% below the previous month (146.9).

"Despite the decline in December, last year was a solid year for truck tonnage, especially during the second half of 2017," said ATA Chief Economist Bob Costello. "I remain optimistic for 2018 for a host of reasons, including a pick-up in factory activity, better housing construction, solid retail sales, and an expected shot in the arm from the new tax law."

Trucking serves as a barometer of the U.S. economy, representing 70.6% 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 2016. Motor carriers collected $676.2 billion, or 79.8% of total revenue earned by all transport modes.

ATA calculates the tonnage index based on surveys from its membership and has been doing so since the 1970s. This is a preliminary figure and subject to change in the final report issued around the 10th day of the month. The report includes month-to-month and year-over-year results, relevant economic comparisons and key financial indicators.