ETHAGERSTOWN, MD — Mack Trucks Inc. is boosting employment at its plants due to strong demand for its heavy-duty trucks, according to a report by the Associated Press on April 14. In the past 15 months, the company added 275 workers at its Hagerstown powertrain factory, bringing employment there to 1,522 by March 31, spokesman Robert Martin said to the Associated Press. During the same period, the company added 213 jobs at its assembly plant in Macungie, Pa., bringing employment there to 975, Martin said. Employment at Mack's New River Valley plant in Dublin, Va., has risen to 3,000 from 2,200 in the past year, spokesman James McNamara said. Martin said the employment gains reflected increased sales of Mack trucks, from 15,146 units in 2003 to 20,670 last year, a 36 percent jump. He said sales are rebounding from a slump earlier in the decade linked to general weakness in the economy and hesitation among customers to be the first buyers of trucks with less-polluting diesel engines required by the federal Environmental Protection Agency starting in October 2002. Some customers were fearful the change would mean either lower gas mileage or less power, so those who didn't need to increase their fleets during the economic downturn waited for others to try out the engines first. Although the new engines are slightly less fuel-efficient, Mack has helped its customers train drivers to get the most from the motors, Martin said.