The market for medium- and heavy-duty trucks in the U.S. lost some steam in January from December’s 40.6% rise, but nevertheless begins 2012 a healthy 35.9% ahead of year-ago, WardsAuto data shows.

Big-truck makers delivered 24,471 units during the month, compared with 18,001 prior-year.

As was the case throughout most of 2011, Class 8 trucks powered the gain, although other classes saw increased sales as well.

Class 8 deliveries rose 52.9% from like-2011, as all but Volvo Truck’s namesake brand, down 5.2%, posted gains.

Daimler increased its segment-topping lead over International, but both companies saw their Class 8 market shares chipped away by PACCAR, whose Kenworth brand posted the biggest rise, at 93.2%.

Class 7 sales jumped 14.2%, with all manufacturers but International (off 12.0%) recording increases. PACCAR’s Peterbilt brand led all gainers, up 192.6%.

International’s falloff allowed Freightliner to assume Class 7 market-share and volume leadership, as it outsold its rival 1,331 to 1,274 in the month.

A hefty decline in volume at Ford (down 72.5%) was offset by increases elsewhere in Class 6, including PACCAR’s 223.1% jump, boosting segment sales 19.0%.

Class 5 deliveries ballooned 27.6% in January. Most of that gain came from Chrysler, whose sales spiked 291.6% to 885 units. The company’s market share rose to 26.6% from 8.7% year-ago, eating into Ford’s leadership. Ford recorded a 59.9% Class 5 share last month, compared with 75.4% in January 2011.

Isuzu, No.2 in Class 5 market share year-ago, took the biggest tumble in the sector, with sales down 39.5%.

As in December, Class 4 was the lone group to lose volume, off 11.2% from like-2011. Isuzu’s whopping 866.7% increase in domestic-model sales to 116 units wasn’t enough to counter Ford’s 94.7% plunge and International’s 30% decline.

Class 8 days’ supply in January inched up to 59 from 58 year-ago, while actual stocks rose 12,000 units, to 34,751.

Medium-duty days’ supply and inventory jumped equally, with 82 days’ (35,161 units) worth of trucks at the end of last month, vs. 69 days’ (25,031) year-ago.