Sales of medium- and heavy-duty trucks climbed 16.7% in May, the smallest sales increase of the year compared with like-2011 and paling in comparison with April’s 38.9% surge.

A 24.6% plunge in Class 6 deliveries and an 8.3% uptick for Class 7 tempered the strong growth of Classes 4, 5 and 8 last month, WardsAuto data shows.

Class 8 saw a 25.1% increase across the entire segment. Daimler’s Western Star led with sales soaring 95.6%, while the Volvo Truck brand’s 14.3% rise was the smallest gain in the group.

Medium-duty deliveries grew 7.3%. Class 7’s No.2 volume-seller, International, suffered a 16.9% decline, but Hino’s 95.5% surge put the group in the black.

All Daimler brands, Ford and PACCAR’s Peterbilt saw Class 6 sales slide last month. Daimler’s Mitsubishi Fuso deliveries plunged 84.6% on minor volume, while Freightliner sales tumbled 54.4%. Kenworth posted the biggest gain, up 58.2%.

The situation was better for the two Daimler brands in Class 5, where a whopping 524.9% spike at Freightliner and 152.6% increase by Mitsubishi Fuso drove up group deliveries 35.2% from year-ago.

Class 4’s 44.4% sales gain was the largest of any medium- or heavy-truck group in May. Despite’s losses by Ford, down 96.4%, and Daimler, off 29.2%, strong performances by the remaining sellers pushed the group into positive territory. Sales of Isuzu imported models jumped 81.7% and International’s climbed 66.6%.

Through May, medium- and heavy-duty truck deliveries were up 29.1% to 143,393 units, compared with prior-year’s 111,061.

In other big-truck news:

  • Freightliner touts the fuel efficiency of its new ’14 Cascadia Evolution model. The Class-8 truck powered by a Detroit DD15 engine and DT12 automated manual transmission, and boasting aerodynamic improvements such as a new air dam and bumper closure, averaged 10.67 mpg (22.0 L/100 km) on an 8.5-mile (13.7-km) closed-course Texas track. To earn that number, the Evolution drove for 1,000 miles (1,609 km) nonstop at an average speed of 60 mph (97 km/h), with a gross cargo weight of 76,000 lbs. (34,473 kg), Freightliner says.
  • The U.S. Court of Appeals in Washington says a U.S. Environmental Protection Agency interim rule that allowed Navistar to sell diesel engines emitting more than 0.20g nitrides of oxygen was erroneous.The EPA had been allowing Navistar to circumvent 2010 compliance rules if the truck maker paid a fine. In a June 13 ruling, the court says the interim EPA rule violated the Administrative Procedures Act, as it allowed no opportunity for comment, Bloomberg reports.Mack Truck and Volvo Truck brought the lawsuit against the EPA. Navistar says it disagrees with the court ruling and is seeking a rehearing.

cschweinsberg@wardsauto.com