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-percent rise from like-2011, according to research by WardsAuto.com.

Most of the 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%.

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

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

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

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

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.