Global truck sales should expand by 4.3% this year over the 2012 level, boosted by medium-duty models and nations with developing economies, according to a report from consulting firm Frost & Sullivan.

Medium-duty U.S. retail truck sales rose 13.6% in January above the same month in 2012, the 38th consecutive month of improvement, WardsAuto.com said.

Retail heavy-duty truck sales in the United States fell 15.6% in November from a year ago, the third straight monthly decline, but continue to outpace 2011, WardsAuto.com reported.

Medium-duty U.S. retail truck sales increased 5.9% in September from a year earlier, as Classes 4-5 surged, but Class 6 sales slumped, WardsAuto.com reported.

Class 8 truck sales rose 19.4% in June over the year-ago level, extending a growth pattern that dealers and analysts said could be in jeopardy during the rest of 2012 because of growing economic uncertainty.

Medium-duty engines over the past decade have gotten a lot smarter, thanks to the advanced electronics developed first in heavy-duty engines.

Recovery in the medium-duty truck market gathered strength in March, as overall U.S. retail sales in Classes 3-7 increased for the fourth consecutive month, according to WardsAuto.com.