U.S. big-truck sales soared in May, up 61.9% vs. year-ago and far surpassing April’s 31.2% increase, previously 2011’s best monthly performance.

Class 8 sales made the biggest leap, up 82.4% thanks to triple-digit percentage increases at Volvo Truck and PACCAR, up 175.4% and 145.4%, respectively.

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

Medium-duty sales grew 43.8% last month, with Class 7 up 40% on gains at all but three manufacturers. Ford’s 197.4% surge was the group’s biggest year-over-year increase, while Isuzu’s 72.9% plunge marked the biggest loss.

Class 6 managed a 45.5% increase over like-2010, with Daimler’s Freightliner brand skyrocketing 213.0%. That gain helped offset the 31.6% decline at Daimler’s Mitsubishi Fuso, the segment’s biggest May falloff.

Despite declines at many manufacturers, Class 5 recorded the second-highest increase last month, up 73.1% thanks to Chrysler’s whopping 391.1% gain. Ford, however, retains its spot as the segment’s market-share leader with 56.6%.

Hino’s sales plummeted 86.5% in May for Class 5’s biggest tumble.

Class 4 was the only medium-truck group in the red for the month. Sales dipped 9.9% compared with like-2010 as UD Trucks, International and Isuzu’s domestically built units recorded sharp losses.

Mitsubishi Fuso was the top Class 4 performer, up 84.7%.

Through May, sales of medium- and heavy-duty trucks in the U.S. were tracking 30.9% ahead of like-2010.

As was the case in April, units in inventory rose, but days’ supply slipped for both Class 8 and medium-duty.

Class 8 held 26,420 units in stock at the end of May, up from 20,788 year-ago. Days’ supply fell 20 days, to 48.

Medium-duty stock swelled to 30,060, compared with like-2010’s 27,630. Days’ supply also fell 20 days, to 61.

In other big-truck news, Daimler says it will add output and jobs at its North American plants this year.

In the second half the company will hire about 1,230 workers at its Mt. Holly and Gastonia, NC, plants, as well as at the truck maker’s sites in Portland, OR, and Saltillo, Mexico.

The Mt. Holly, Saltillo and Portland employment increases are due to demand for Western Star and Freightliner trucks, while Gastonia is gearing up to increase parts’ supply.

Daimler claims a 31.6% share of the U.S. market’s Class 6, 7 and 8 truck sales.

cschweinsberg@wardsauto.com