Mack Trucks Inc. yesterday reported a loss of $59.2 million ($2.07 a share) for 1985, compared with a net gain of $74.9 million ($2.29) for the previous year.

Mack attributed the losses to a $68.3 million write-off charge stemming largely from the company's planned shutdown of its 60-year-old Allentown, Pa., assembly plant and construction of a new plant in South Carolina.

The closing of the old plant is part of a four-year restructuring plan that also includes implementation of productivity improvement programs and contracting with other companies for certain truck components now produced at Mack plants, company officials said.

Mack reported 1985 net sales of $2.06 billion, down 2 percent from record 1984 sales of $2.1 billion.

Mack lost $2.99 million (13 cents) during the fourth quarter of 1985, compared with income of $19.1 million (60 cents) during the same period last year. Sales during the fourth quarter of 1985 were $490.9 million, a 17 percent drop from $590.7 million.

*Fruehauf Corp. reported a 17 percent decline in net income and an 8 percent drop in overall sales for 1985 from record 1984 levels because of reduced demand in the truck trailer business.

Net earnings for the year ended Dec. 31 totaled $70.5 million ($3.52 a share) compared with $85.2 million ($4.31) the year before. Sales declined to $2.56 billion from $2.78 billion the year before.

For the fourth quarter ended Dec. 31, net income fell 19 percent to $18.2 million (91 cents) from $22.5 million ($1.14) in the final quarter of 1984. Sales declined 6 percent to $636 million from $678.3 million.