It was another disappointing month for big-truck sales in the U.S.

Declines in all groups but Class 4 led to a 16.2% slide in total heavy- and medium-duty deliveries in March, WardsAuto data shows.

Class 8’s 21.5% drop from year-ago was the largest and maintains the trend set down in recent months.

While Daimler’s Western Star posted a 63.0% increase, it was on small volumes and not enough to offset declines at other Class 8 manufacturers.

Volvo Truck suffered the biggest plunge in heavy-truck sales in March, down 45.9%, due to the 60.5% falloff in its namesake brand.

Medium-duty deliveries were more resilient, sliding just 10%, with a 33.8% Class 4 hike offsetting some of the losses for Classes 5, 6 and 7.

Class 7 sales tumbled 24.1%, with UD Trucks and International each recording declines of about 60%.

Hino enjoyed the best growth in the month, up 56.4%, but on just 175 units. PACCAR sales rose 14.6%.

Class 6 declined a more modest 2.1%. International’s 48.6% plunge was the biggest in the group, and enough for Daimler to overtake the beleaguered Illinois-based manufacturer as the Class 6 volume leader.

Daimler’s share of group sales grew from 37.8% in March 2012 to 42.4% last month, while International saw its 42.6% year-ago share dwindle to 22.3%.

Ford was the most improved Class 6 manufacturer in the month, with deliveries up a whopping 390.9% to 852 units.

Class 5 sales slid 8.9%, with Chrysler deliveries more than halved from like-2012. Ford strengthened its hold on the sector, with its share rising to 74.8% in the month from 61.1% year-ago.

Ford’s 216 units, up from just 10 year-ago, powered the Class 4 gain. Also performing well in the group was Isuzu, with sales of its domestically built models jumping 41.2%.

Mitsubishi Fuso was the only Class 4 truck maker in the red, off 50.4% from year-ago.

Through March, sales of medium- and heavy-duty trucks were running 9.3% behind like-2012, with 75,423 units vs. 83,181.

Inventory declined but days’ supply rose for both medium and heavy models last month. There were 33,193 Class 8 trucks in stock in the U.S. at the end of March, a 68-days' supply.

Year-ago, 39,647 units were in stock, equivalent to 64 days’ supply.

Medium-duty inventory fell to 35,355 units from 36,329 in late March 2012, but days’ supply rose to 76 from 70.

In other big-truck new, a pension fund reportedly is suing International-maker Navistar over what it calls misleading statements made about diesel engine emissions-control technology later abandoned by the company.

The Norfolk Country Retirement System says former CEO Dan Ustian and current Chief Financial Officer Andrew Cederoth concealed troubles Navistar was having meeting clean-air requirements with an exhaust-gas recirculation system on its diesel engines, Bloomberg reports.

Navistar was banking on the technology to meet U.S. Environmental Protection Agency emissions standards but it failed to do so, causing the manufacturer to pay non-compliance fines before switching to selective catalytic reduction technology, a urea-based after-treatment system used by competitors.

The pension plan is requesting unspecified compensation for investors holding Navistar stock between June 9, 2010, and August 1, 2012.

Separately, Navistar has ceased production at its Garland, TX, plant. The facility will close for good May 31, displacing 900 workers, says the website of Dallas TV station NBCDFW.

The Garland closure was announced by Navistar last fall as part of a move to consolidate production in Ohio and Mexico.