ARLINGTON, Va., Jan. 22, 2013 /PRNewswire-USNewswire/ -- The American Trucking Associations' advanced seasonally adjusted (SA) For-Hire Truck Tonnage Index increased 2.8% in December after surging 3.9% in November. (The 3.9% gain in November was revised from a 3.7% increase ATA reported on December 18, 2012.) The back-to-back increases in November and December were by far the best of gains of 2012. As a result, the SA index equaled 121.6 (2000=100) in December versus 118.3 in November. Despite the solid monthly increase, compared with December 2011, the SA index was off 2.3%, the worst year-over-year result since November 2009. For all of 2012, tonnage was up 2.3%. In 2011, the index increased 5.8%. (Logo: http://photos.prnewswire.com/prnh/20100129/ATALOGO) The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 110.3 in December, which was 4.9% below the previous month. "December was better than anticipated in light of the very difficult year-over-year comparison," ATA Chief Economist Bob Costello said. In December 2011, the index surged 6.4% from the previous month. Costello anticipates more sluggishness in the index this year, especially early in the year, as the economy continues to face several headwinds. "As paychecks shrink for all households due to higher taxes, I'm expecting a weak first quarter for tonnage and the broader economy," Costello said. "Since trucks account for the vast majority of deliveries in the retail supply chain, any reduction in consumer spending will have ramifications on truck tonnage levels." Note on the impact of trucking company failures on the index: Each month, ATA asks its membership the amount of tonnage each carrier hauled, including all types of freight. The indexes are calculated based on those responses. The sample includes an array of trucking companies, ranging from small fleets to multi-billion dollar carriers. When a company in the sample fails, we include its final month of operation and zero it out for the following month, with the assumption that the remaining carriers pick up that freight. As a result, it is close to a net wash and does not end up in a false increase. Nevertheless, some carriers are picking up freight from failures and it may have boosted the index. Due to our correction mentioned above however, it should be limited. Trucking serves as a barometer of the U.S. economy, representing 67% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled 9.2 billion tons of freight in 2011. Motor carriers collected $603.9 billion, or 80.9% 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.