Makers of medium- and heavy-duty trucks notch their biggest year-over-year increase in U.S. sales so far this year, as volume of 28,437 units in April marked a 38.9% rise from like-2011.

Most of that gain came from the Class 8 sector, which saw a 56.2% jump to 16,905 trucks. But Classes 4-7 also recorded a healthy rise, combining for 11,532 units, up 19.4%.

All Class 8 manufacturers saw high double-digit increases in April, with International’s 32.9% uptick the lowest in the segment. Both PACCAR and Daimler notched 60%-plus gains, accounting for a majority of the segment’s volume growth in the month.

Class 7 accounted for the bulk of the medium-duty sector’s rise, with sales up 45.4% to 4,257 units. Much of that increase came from Daimler, which posted a 76.8% gain to 1,826, but most other makers also recorded strong performances. Only UD, whose sales fell to just 15 trucks, suffered a decline.

Class 5 also finished the month in positive territory, with sales rising 21.6% on volume of 3,560. Daimler, International and Isuzu were responsible for most of that growth.

Class 4 sales were relatively flat, down less than 1% to 706 units, while Class 6 deliveries dipped 2.6% to 3,009. Daimler (down 20.4%) and Ford (off 74.4%) suffered the most from the decline in Class 6 demand.

The April performance, the 28th straight month of year-over-year gains, kept the big-truck market 29.9% ahead of like-2011 for the year. That’s down from a 32.3% clip through the first quarter, as comparisons with year-ago begin to get tougher.

The industry closed April with 40,482 Class 8 trucks in stock, equal to a 57-day supply. That compares with 56 days’ supply and just 25,338 units year-ago.

Medium-duty inventories swelled to an 86-day supply on 41,376 units, from 70 days’ and 36,329 units at the end of March. Year-ago, stocks stood at 29,573 Class 4-6 trucks, good for a 74-day supply.

In other big-truck news, Western Star now is offering a 15.6L Detroit Diesel engine on its 6900 truck model. The engine, which generates 600 hp and 2,050 lb.-ft. (2,780 Nm) of torque, is ideal for extreme vocational applications, including logging, mining and heavy haul, the truck maker says.

Western Star also marks the 10th anniversary of its Portland, OR, plant. The facility, which employs more than 1,000 people on two shifts, produces the 4700, 4800, 4900 and 6900 models.