Medium- and heavy-duty U.S. truck sales soared 31.2% in April, the sector’s best performance yet in 2011.

The April gain tops February’s 30% year-on-year increase, according to Ward’s data.

Related document: Ward’s U.S. Truck Sales by Weight Class – April 2011

Class 4 was the only segment to decline. Deliveries plunged 50.1% as Daimler and International posted losses of 70.2% and 78.5%, respectively.

Class 8 sales jumped 35.9% in April vs. like-2010, propelled by across-the-board gains. Volvo led the pack with a 113.1% boost, followed by a 104.2% surge by PACCAR’s Kenworth brand.

Total medium-duty sales rose 26.3%, with Class 7 posting a 23.5% climb from April 2010. Ford’s 171.3% gain was the group’s biggest, while Isuzu, down 67.9%, was just one of two Class 7 manufacturers to suffer a decline.

Class 6 sales soared 50.3% in April vs. year-ago, with Ford again leading. Blue Oval deliveries skyrocketed 376.4% on 658 deliveries, up from year-ago’s 133.

As in Class 7, losses were few, with Kenworth’s Peterbilt and Daimler’s Mitsubishi Fuso posting declines of 15.0% and 8.1%, respectively.

Class 5’s 63.1% gain was the largest of any medium-duty group, even though more than half the brands posted declines. Segment-leader Ford strengthened its grip on the No.1 spot. Its 130.5% hike more than doubled like-2010.

But it was Chrysler, with a 146.5% jump that led all Class 5 brands posting increases.

Daimler’s Freightliner posted an 85% loss.

Through April, sales of medium- and heavy-duty trucks in the U.S. are running 26.3% ahead of like-2010.

Units in inventory rose, but days’ supply fell for both the Class 8 and medium-duty sectors.

Class 8 days’ supply at the end of April totaled 56, down from 65 year-ago, but stocks tallied 25,338 vs. 21,485.

Medium-duty days’ supply tumbled to 74 from 88, but inventory grew to 29,573 from year-ago’s 28,021.

In other big-truck news, Daimler’s truck division reportedly tripled its operating profit in the first quarter, to E415 million ($589 million) on sales of 89,000 units, a 27% increase from Q1 2010.

Other truck makers experienced similar growth, with PACCAR’s earnings up to $193.3 million in Q1 compared with $68.3 million year-ago.

cschweinsberg@wardsauto.com