Heavy-truck makers ended 1997 with a bang as December sales soared 33.5 percent, and 1998 may be nothing to whimper about either.

The December gain from the year-earlier period came on sales of 18,748 heavy trucks, a surprisingly strong finish to a year that started with predictions of a double-digit decline.Instead, 1997 closed with a 5 percent rise as U.S. retail sales reached 178,551 vehicles, according to figures from the American Automobile Manufacturers Association. It was the third-biggest year in the industry's history after 1994 and 1955.

WEFA Inc., an Eddystone, Pa.-based forecasting firm, said that conditions are right for continuing growth this year. Sales of 180,000 trucks are expected in the heavy, or class 8, category, defined by gross vehicle weight ratings of over 33,000 pounds.

Thanks to a healthy economy and capacity shortages among carriers, truck makers are carrying a huge order backlog into 1998 that will keep them busy into the third quarter, WEFA said. While traffic growth is expected to ease, other factors like low fuel prices point to another good year.

Double-digit December gains also pushed most makers into positive territory for 1997.

Freightliner Corp. of Portland, Ore., ended the year in first place, with December sales of 5,140 heavy trucks, a 31.6 percent jump from 1996. Sales of 50,282 vehicles gave the firm an 0.7 percent increase for the year and a 28.2 percent market share. The recent Freightliner acquisition of Ford Motor Co. heavy-truck operations, renamed Sterling Truck Corp., produced a combined market share of 35.2 percent. December sales of 1,143 trucks with Ford nameplates fell 12.4 percent, while 1997 deliveries of 12,645 vehicles dropped 17.4 percent, rounding down to a 7.1 percent share. Combined Freightliner-Ford sales of 6,283 trucks rose 20.5 percent in December, but 1997 totals of 62,927 units were down 3.5 percent.

Paccar Inc. of Bellevue, Wash., placed second at 21.4 percent share, based on returns from its two divisions, Peterbilt Motors Co. of Denton, Texas, and Kenworth Truck Co. of Kirkland, Wash. Peterbilt sales of 2,004 trucks were up 13.4 percent last month, pushing 1997 sales of 20,277 units, to a 2.9 percent gain and an 11.4 percent market share.

Kenworth's monthly sales of 1,700 trucks climbed 22.8 percent, as sales of 17,988 vehicles rose 1.8 percent last year, yielding a 10.1 percent share. Combined Paccar sales of 3,704 units advanced 17.5 percent in December, while 1997 deliveries of 38,265 trucks rose 2.4 percent.

Sales jumped 75.9 percent last month, to 3,569 vehicles, at Navistar International Transportation Corp., giving the Chicago-based manufacturer 19.3 percent of the market, as 1997 deliveries of 34,404 trucks rose 21.3 percent.

December sales of 2,925 vehicles climbed 30 percent at Mack Trucks Inc. of Allentown, Pa., driving 1997 totals of 22,375 trucks up 8.7 percent for a 12.5 percent market share.

Volvo Truck North America Inc. of Greensboro, N.C., captured 9.7 percent of the market, as December sales of 2,018 vehicles posted a 66.9 percent increase, hiking 1997 deliveries of 17,292 units by 11.9 percent.