U.S. medium- and heavy-duty-truck sales posted a 22.7% increase in August over year-ago, marking the month as 2010’s best so far.

Class 6 led the pack with a 57.3% gain on like-2009, while Class 5 and 8 also saw large increases. Class 7 posted a modest rise in sales, and Class 4 was the sole loser in the month, down 52.7%.

Related document: Ward’s U.S. Truck Sales by Weight Class – Aug. 2010

Nearly all Class 8 manufacturers saw increases. Daimler AG’s Western Star brand led with deliveries surging 173% to 126 units, compared with 48 year-ago. Freightliner deliveries grew 46.3% to 2,630, edging out International Truck and Engine Corp. by two units and making Daimler the best-selling Class 8 manufacturer in August.

Medium-duty sales rose 17.2%, as Class 4’s hefty 52.7% decline tempered the large increases of Classes 5 and 6.

Class 7’s relatively meager 5.7% uptick in August was due to losses at about half the group’s manufacturers. While volume-leading Daimler posted a 40.2% spike, thanks almost entirely to Freightliner’s 47.6% group-leading jump, moderate-volume brands such as Daimler’s Sterling (down 97.7%) and General Motors Co. (down 97.2%) stumbled.

Class 6’s 57.3% ascent was thanks to gains of 36% or more by nearly all manufacturers in the group. PACCAR Inc.’s Peterbilt brand’s 350.7% rise on modest volume of 26 units vs. six year-ago, was the largest increase.

However, Ford Motor Co. climbed 304.8% on larger volumes of 432, compared with 111 in like-2009. Repeating its Class 7 performance, Daimler’s Sterling was the biggest loser in Class 6, down 87.4% from prior-year with just four trucks sold.

Ford was the biggest gainer in Class 5, with sales of 1,755 trucks, good for a 197.7% hike over year-ago and allowing the company to remain the group’s volume leader.

GM’s 93.7% sales drop in domestically built units was Class 5’s largest decline. Other manufacturers posted losses on very small volumes.

Daimler’s Mitsubishi Fuso shone brightest in Class 4 last month, up 65.5%., a rare increase compared with competitors.

A 52.7% drop in Class 4 deliveries was due to GM and Ford. As it exits the medium-duty sector, sales of GM’s domestically built trucks plummeted 90.4%, while its imported models slid 85.1%. Although Ford’s Class 4 deliveries tumbled 76.6%, the truck maker still beat GM with its 123 units vs. 546 year-ago.

Through August, sales of medium- and heavy-duty trucks in the U.S. were up 11.5% from prior- year to 139,996 units.

Class 8 inventory at the end of the month stood at 19,847, or a 56 days’ supply, down from 22,567, or 82 days’ year-ago. Medium-duty stocks fell to 22,253 units, or a 66 days’ supply, from 30,049, or 105 days’, at the close of August 2009.

In other big-truck news, VanDyne SuperTurbo Inc. signs a pact with Cummins Inc. to develop a turbocharger to improve Class 8 fuel economy. The agreement is part of the U.S. Department of Energy’s Super Truck program, which in turn is part of the American Recovery and Reinvestment Act to spur technology innovation.

Cummins’ $39 million award under Super Truck is credited to its plan to improve vehicle-freight efficiency 50%, using technologies such as VanDyne’s turbochargers that capture exhaust-waste heat as additional engine power.