U.S. sales of medium- and heavy-duty trucks in May outpaced like-2009 by 16.1% for the year’s largest gain.

April had been the industry’s best performance, until last month, with a 10.8% sales boost.

Credit Class 6 deliveries for the May increase, with 3,071 deliveries more than doubling prior-year’s 1,304.

Related document: U.S. Truck Sales by Inventory and Weight Class – May 2010

Volume-leader International Truck and Engine Corp. accounted for most of the hike, with 1,715 sales, more than twice the 650 chalked up in like-2009.

Ford Motor Co.’s 564-unit May tally was nearly five times the number of sales it garnered year-ago, while Daimler AG’s Freightliner brand realized a 99.2% boost on 512 deliveries.

Class 5 trucks marked a 27.4% increase with 2,165 deliveries. It was the only other medium-duty segment in positive territory.

Volume-leader Ford and Isuzu Motors Ltd. were the big winners. Ford’s 1,457 sales were good for a 91.2% gain over like-2009, while Isuzu marked a second consecutive strong month tripling prior-year’s deliveries to 203 units.

Class 7 sales totaled 2,647 for an 8.3% shortfall. International suffered a 14.2% drop to 850 units, while PACCAR Inc. took a 46.6% hit on less volume.

PACCAR’s Kenworth and Peterbilt brands delivered 100 and 145 units, respectively, compared with year-ago’s 224 and 235.

The Class 7 picture might have been worse if not for volume-leading Freightliner’s 22.6% increase on 1,107 deliveries.

Hino Motor Mfg. USA Inc. also stemmed the decline. Its 157-unit tally was more than five times the number of sales the truck maker recorded in like-2009.

Class 8 sales jumped 13.5% to 7,895 deliveries. International led the way with a 34.1% surge to 2,684 unit-sales.

Freightliner saw an 18.8% increase to 2,233 deliveries, compared with prior-year. Kenworth recorded a 19.3% sales boost, but Peterbilt’s 9.8% decline trimmed parent PACCAR’s gain to 4.2%.

In a similar situation, Volvo Truck North America Inc.’s Class 8 May sales slipped into negative territory. Volvo’s Mack brand delivered 626 units for a 5.6% gain, compared with like-2009. But the truck maker’s core brand saw sales plunge 10.4% to 459.

Class 4 lagged the entire big-truck industry on a volume and percentage basis. The segment’s deliveries sank 35.7%, compared with year-ago, to 1,070 units.

Ford and Freightliner were the primary culprits, with freefalls of 75.8% and 86.4%, respectively. Ford tallied 192 sales, compared with prior-year’s 794, while Freightliner totaled 44 compared with 324 in like-2009.

Obscured by the bad news were strong performances by International and Daimler’s Mitsubishi Fuso brand. International’s 115 deliveries were nearly six times greater than year-ago’s total, while Mitsubishi Fuso’s 105 sales almost tripled.

Despite Class 4’s poor showing, medium-duty sales totaled 8,953 in May for an 18.5% improvement over prior-year.

Through May, medium-duty sales were tracking 5.9% ahead of like-2009. Total medium- and heavy-duty sales through May led year-ago by 9.6%.