The medium- and heavy-duty truck market, replaying the scenario common in every month of 2011, soared in December, powered by a huge spike in Class 8 sales.

December’s 46.3% jump was the fifth-biggest monthly gain of 2011, WardsAuto data shows.

For the year, sales of big trucks in the U.S. rose 40.6%, to 306,189 units, from 2010’s 217,702. All but Class 4 posted gains in 2011.

Class 8 grew the most for the month and the year, up a whopping 78.3% in December, and 59.9% for 2011.

Every Class 8 manufacturer posted an increase last month, including Volvo Truck, whose namesake brand was up 170.1% from like-2010. The Volvo brand also was the segment’s leading gainer in 2011, up 108.8%.

International posted the most modest increase of all Class 8 brands in 2011, up 33.4%.

Daimler retained its title as Class 8 volume leader, selling 54,367, mostly Freightliner, units last year.

Total medium-duty sales rose 12.2% in December, and 22.0% in 2011.

Among gainers, Class 7’s growth was the smallest, with sales up 7.5% for the year. Segment volume slipped 2.6% below year-ago in December, as a 153.2% increase in Hino sales failed to offset declines at International and Ford.

PACCAR’s Peterbilt notched the biggest rise in Class 7 for 2011, 48.1%, while an exiting General Motors lost 99.8% of its 2010 volume. International remained Class 7’s volume leader, although Ford and PACCAR chipped away at its market share.

Class 6 sales grew more than 30% both for the month and year overall, with most brands in December posting triple-digit percentage gains, including Daimler’s Freightliner, up 182.4%.

Freightliner also grew the most for the year, with segment sales up 125.2%. Parent Daimler gained 11.3 points of market share for the year, taken mostly from Class-6 volume-leader International.

GM, abandoning another medium-duty segment, saw its Class 6 deliveries plunge 96.2%.

Class 5 grew 17.1% in December, with a 322.6% increase at Daimler’s Mitsubishi Fuso outpacing mild declines at Isuzu and Chrysler.

For 2011, Class 5 sales rose 37.1%, with Ford widening its segment lead to a 64.3% share from 2010’s 59.1%. But International grew the most year-on-year in volume, up 153.4% to 3,831 units.

Class 4 was the only group to lose ground in 2011, with sales off 13.4% due to 97%-plus drops at GM and Hino, both exiting the sector, and declines at International and Ford.

Isuzu again held Class 4’s top spot, selling more than 5,000 domestically built and imported trucks in 2011.

In December, Class 4 sales rose 2.6%, with triple-digit percentage increases posted by several manufacturers.

Class 8 inventories rose at the end of December, up roughly 10,000 units vs. like-2010. However, days’ supply slipped to 41 from 49 year-ago.

Medium-duty inventory also spiked at the end of the year, with 77 days’ supply on 35,081 trucks on Dec. 31, up from like-2010’s 57 days and 23,283.

In other big-truck news, Daimler says it will add 1,100 new jobs at its Cleveland, NC, plant by the end of 2012, as it goes to a second shift. Employment at the Freightliner Class 8 truck plant will grow 72% above current levels, with an additional 1,072 shop workers and 29 engineering and support staff forecast.

Most jobs will be filled by workers laid off in 2009, Daimler says. Daily production rates at the plant are expected to double by October, and Daimler says it has at times had a 6-month backlog of orders of its Cascadia model to fill at its largest U.S. plant.

The truck maker also will be adding 100 jobs this year at its nearby Gastonia, NC, logistics plant.