Sales of medium- and heavy-duty trucks in the U.S. rose 1.5% in April, the first year-on-year gain in 2013.

Class 8 sales fell 11.1% on declines at International, down 34.2%, and PACCAR, which dropped 22.6% on slowdowns by both the Kenworth and Peterbilt brands. The leading gainer in Class 8 in April was Daimler's Western Star, climbing 54.9%.

Medium-duty sales fared better, rising 20.0% last month, as Class 7 registered a 2.9% gain and Class 6 grew 37.2%.

In Class 7, International's 25.3% increase was the largest, and offset a 54.4% tumble at Ford, as well as smaller declines at Peterbilt and Hino. Mitsubishi Fuso, exiting the group, was down 4.0%.

Class 6's 37.2% jump, the largest of any big-truck sector in the U.S. in April, could be credited to Ford, which saw a whopping 520.2% hike, boosting the brand's share from 5.0% year-ago to 22.5% last month.

Freightliner's 116.6% gain propelled parent Daimler to a 116.3% spike in Class 6 in April.

The biggest loser in the group was UD Trucks, down 88.5% on just three units sold vs. 25 in April 2012.

The 29.8% rise in Class 5 sales was the second-best gain last month, driven by Chrysler and its 111.3% positive result. Chrysler's 1,116 units sold in April gave it a 23.2% share of Class 5, compared with 14.2% year-ago. Chrysler remained far behind Ford, though, which held a commanding 66.2% share of the group.

PACCAR saw the biggest loss, down 86.3% in Class 5 in April.

The smallest increase of the month was in Class 4, up just 0.5%. Hampering Class 4 were losses at half the group's manufacturers, including a 55.6% slide by Mitsubishi Fuso.

International's 68.0% jump was the largest, and Ford added 283 units to the group's April tally from just eight year-ago.

Through April, sales of medium- and heavy-duty trucks in the U.S. were running 14.6% behind like-2012, with 54,451 units sold compared with 63,750 in the January-April period last year.

Days' supply and units in inventory declined in both the Class 8 and medium-duty sectors last month.

Class 8 ended April with a 55 days' supply, down from 57 year-ago. Total units in inventory fell to 34,420 from 40,482, WardsAuto data shows.

Medium-duty models had a more dramatic decrease in days' supply, falling to 65 days from 86 in the year-ago period. Some 37,394 medium-duty units were in stock in late April, down from 41,376 in like-2012.

In other big-truck news, Navistar International on April 29 began shipping its first ProStar models using its MaxxForce 13.0L diesel engine. The mill uses the truck maker's selective- catalytic-reduction technology.

Navistar is enduring lasting consequences from its prior decision to use an exhaust-gas recirculation system to treat its diesel-engine emissions, including lawsuits from shareholders.

The day before the ProStar announcement, a lawsuit was filed on behalf of Navistar shareholders between Nov. 3, 2010 and Aug. 1, 2012, alleging the company knowingly misled investors about the viability of its decision to use EGR.

The lawsuit accuses Navistar of failing to disclose that its "attempted methods to achieve compliance with Environmental Protection Agency guidelines in truck manufacturing had failed" and "Navistar did not have engines ready to meet the 2010 EPA standards."

cschweinsberg@wardsauto.com