Deliveries of medium- and heavy-duty trucks in the U.S. slid backwards into negative territory in May, down 4.8% following an April increase, WardsAuto data shows.

Classes 4, 7 and 8 were in the red, but all other segments posted gains in the month.

A hefty 41.9% drop in International’s deliveries, plus smaller declines at most of the group’s other manufacturers, pushed Class 8 sales down 14.2% from year-ago.

One of the few improving brands was Daimler’s Western Star, which saw a 39.3% increase in May, compared with year-ago, on just 298 deliveries, the smallest volume in the segment for the month.

Strong performances by Classes 5 and 6 more than balanced the decline in Classes 4 and 7, giving medium-duty sales a 7.4% boost.

UD Trucks suffered the largest drop in Class 7, with deliveries last month plunging 52.4% on small volume. But International, No.2 in volume behind Daimler, made the biggest impact on total segment sales, tumbling 34.4%. Its market share dropped to 21.1% from 29.2% year-ago.

International’s slide, as well as falling sales at Ford, Hino and Kenworth, boosted Daimler’s Class-7 share climb to 51.7% from 41.1% year-ago.

Class 6 enjoyed the largest gain among any big-truck group in the U.S. in May, up 23.7%.

A 406.3% spike in sales by Ford, as well as a 137.4% surge by Daimler, powered the Class-6 increase and offset declines seen at all other manufacturers.

As in Class 7, UD Trucks and International saw the largest declines last month, with International’s loss based on heftier volume.

Class 5 deliveries rose 16.7% in May thanks to Ford, which enjoyed a 35.6% increase from year-ago, strengthening its hold on the group. Its segment share stood at 66.9%, up from 57.5% in like-2012. Mitsubishi Fuso saw the biggest decline in the group, down 28.8%.

Hefty losses by imported Fusos and Isuzu models put Class 4 sales 16.7% behind like-2012. Isuzu’s 46.2% increase in sales of domestic-built models, as well as 385 units sold by Ford, could not offset the imported-model losses.

Deliveries of medium- and heavy-duty trucks in the U.S. through the first five months were down 5.3% from like-2012, to 135,737 units compared with 143,393.

Inventory was down for both Class-8 and medium-duty models in May. Class 8 had 34,140 units in stock at the end of the month, or 57 days’ supply, compared with 41,450, or 60 days’, year-ago. Medium-duty stock of 37,510 units, or 66 days’ supply, compared with 41,056, or 78 days’, in like-2012.

In other big-truck news, Navistar International posted a larger-than-expected loss for the second quarter, with revenue down 23% to $2.5 billion and net income a negative $374 million, or $4.65 per share. This compared with a loss of $2.50 per share in like-2012, Barron’s reports.

The Illinois-based truck maker blames slower-than-expected sales and warranty costs related to its failed experiment with exhaust-gas recirculation technology for the poor performance.

In related news, Navistar has begun manufacturing its DuraStar conventional cab and 9800 cab-over-engines models in Canaos, Rio Grande Do Sul, Brazil, says Truckinginfo.com, taking over from a contract manufacturer. The assembly line has a 5,000-unit capacity.

Meanwhile, a Navistar spokesman tells School Transportation News the truck maker will use Cummins’ selective-catalytic-reduction technology to treat diesel emissions from its medium-duty truck engines.

cschweinsberg@wardsauto.com