Sales of medium- and heavy-trucks in the U.S. rose 51.7% from year-ago in September, although that gain marks a slight decline from August’s 54.2% rise, WardsAuto data shows.

The September hike was powered by strong increases in nearly all groups, including a 69.5% jump in Class 8 sales.

Related document: Ward’s U.S. Truck Sales by Weight Class – September 2011

With the exception of Mack, all Class 8 brands saw at least double-digit percentage increases, with two, Western Star and Volvo, posting triple-digit jumps of 305.3% and 126.9%, respectively.

Daimler was the leading volume manufacturer, selling 5,671 units in September, 5,367 of those through its Freightliner channel.

Total medium-duty sales spiked 32.3%, thanks to big year-on-year gains in Classes 5 and 6.

Class 7 was the lone group to suffer a drop last month, with volume sliding 1.6% on a daily basis due to an 86.3% decline at UD Trucks, 44.5% drop at Ford and 7.2% reduction for volume-leader International. PACCAR’s Kenworth brad posted Class 7’s biggest increase, up 99.3%.

Class 6 was the highest-gaining group in September, up 81.1%, driven by an 86.7% increase in domestically built units.

International, up 104.3%, edged out Daimler as the volume leader, but Daimler enjoyed the biggest percentage hike in Class 6, as sales soared 271.0% due to Freightliner’s quadruple-plus volume increase.

Freightliner delivered 1,550 units in September, up 345.4% from like-2010’s 348. Class 6 sales of Daimler’s defunct Sterling brand tumbled 83.3%; while Ford deliveries plunged 81.7%.

Class 5 sales rose 39.3%, with Chrysler the leading gainer, up 123.7%. Volume-leader Ford saw a 30.3% increase from year-ago.

Class 4 sales climbed 10.3% last month, with a 186.8% spike in domestically built Isuzus offsetting declines at other manufacturers.

After months of stellar sales, medium- and heavy-duty trucks through September were pacing 37.4% ahead of like-2010. Days’ supplies dipped slightly last month for both sectors but stocks swelled.

Class 8 days’ supply stood at 48 vs. 52 year-ago, while medium-duty inventories slipped to 68 days from 71.

Each sector had a little more than 30,000 units in stock at the end of last month, vs. roughly 20,000 in like-2010.

In other big-truck news: PACCAR opens a new technical center in Pune, India, which the company expects eventually to employ about 200 people.

The new operation, in collaboration with technology-solutions company KPIT Cummins Infosystems, will have engineering, information technology and component-sourcing responsibilities for PACCAR’s worldwide production and aftermarket operations.

cschweinsberg@wardsauto.com