Medium- and heavy-duty trucks continue their 2011 hot streak, with July sales up 31.1.2% vs. year-ago, Ward’s data shows.

However, July’s performance marks a falloff from the 37.8% increase registered by the sector in June.

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

Class 8 deliveries jumped 39.5% from year-ago, with all manufacturers enjoying gains.

PACCAR’s Peterbilt brand posted the biggest spike in the heavy-duty sector, up 169.7%, while International sales slumped 5.6%.

As in June, all medium-duty classes recorded gains from like-2010, with Class 4’s 130.1% jump far and away the biggest.

Daimler accounted for the bulk of the Class 4 expansion, enjoying triple-digit increases with its Freightliner (661.5%) and Mitsubishi Fuso (786.5%) brands. The truck maker grabbed a commanding 43.3% share of the sector during the month, up from 12.1% year-ago when it trailed both Isuzu and Ford.

Class 7 sales rose 16.1% in July, but some manufacturers suffered declines. Ford’s 204.8% surge was Class 7’s largest increase, while Daimler recorded the biggest drop, down 26.5%.

Class 6 deliveries climbed 21.6% from like-2010. A 23.8% increase in domestic-model sales offset a 55.0% plunge in imports, highlighted by declines at Mitsubishi Fuso (72.0%) and UD Trucks (42.0%). Hino’s 87.8% increase was the biggest in Class 6.

Class 5 sales grew the least, at 5.4%, as a 9.1% rise in domestic-model sales barely offset the 25.3% decline in import deliveries.

International had a huge month, with Class 5 sales soaring 279.0%, taking the No.2 spot in market share from Chrysler. However, it remained well behind Ford, which controlled 53.2% of sales in July. UD Trucks’ 83.8% plunge was Class 5’s largest.

Through July, sales of medium- and heavy-duty trucks were running 32.1% ahead of year-ago. Total deliveries through July tallied 162,191, compared with 122,758 in like-2010.

Units in inventory and days’ supply were up vs. year-ago for both heavy- and medium-duty trucks at the end of July.

There were 27,673 Class 8 units in inventory, or 56 days’ supply, on July 31, compared with 20,010 units (56 days’) in like-2010.

Medium-duty stock swelled to 30,395 units, or 73 days’ supply, from 22,118 (65 days’) year-ago.

In other big-truck news, President Obama announces the country’s first-ever fuel-economy and emissions rules for medium- and heavy duty trucks.

Taking effect for the ʼ14-ʼ18 model years, the new rules will result in savings of 530 million barrels of oil and $50 billion in fuel costs, as well as cut greenhouse-gas emissions by 298 million tons (270 million t), Obama says.

The rules apply differently to the three vehicle categories – semi-trucks; heavy-duty pickups and vans; and vocational vehicles (including buses, garbage trucks and other work vehicles).

For example, semi-trucks are required to reduce fuel consumption and greenhouse-gas emissions roughly 20% by ’18, while vocational vehicles will need to achieve a 10% reduction.

cschweinsberg@wardsauto.com