Strong internal growth paired with new business from a failed competitor helped NW Transport Services Inc. post gains of nearly 30 percent in fourth quarter revenue and tonnage, according to financial reports on file with the Interstate Commerce Commission.

NW Transport, also known as Northwest, posted the best operating results of the six companies included in this week's review of motor carrier financial performance.The Commerce City, Colo., carrier, which hauls freight principally in less-than-truckload lots, showed a fourth quarter revenue gain of 30 percent over the year before. Truck tonnage during the same period rose 28 percent.

The company's operating ratio, its expenses as a share of revenue, was 90.4 percent, among the best for the nation's less-than-truckload fleets.

Even so, NW's operating ratio deteriorated more than six points from the year before as the company had difficulty absorbing the new business.

NW Transport is one of six companies included in the adjoining tables on motor carrier operating and financial results for the fourth quarter of 1987 and for the full year.

The Journal of Commerce publishes statistical results for the nation's 50 largest truck lines shortly after they report to the commission.

Eight major companies, including top fleets like Yellow Freight System Inc., Roadway Express Inc. and Consolidated Freightways Inc., were included in an earlier financial report.

Additional statistical reviews will appear later this week.

Other motor carriers reviewed in this week's tables include:

* Chemical Leaman Tank Lines Inc., of Exton, Pa., one of the nation's largest bulk commodities carriers;

* Southeastern Freight Lines Inc. of Columbia, S.C., a general freight truck line;

* Jack Cooper Transportation Co., Oklahoma City, Okla., an automobile transporter;

* FFE Transportation Services Inc. of Dallas, the country's largest less- than-truckload hauler of temperature-controlled products;

* Jones Motor Co. Inc., Spring City, Pa., a general freight fleet.

Jerry McMorris, president of NW Transport, said a portion of the company's strong tonnage and revenue gains came from Milne Truck Lines Inc., Pleasonton, Calif., which shut down operations last year.

The two companies have overlapping operations in some Western states, although NW serves points as far east as Ohio.

Mr. McMorris said the added business from Milne coupled with NW's own growth stretched the company's capacity in the fourth quarter.

Certainly we had some higher costs related to (employee) overtime that resulted from exceeding capacity levels, but we got that corrected, he said. NW Transport posted an operating ratio of 84.1 percent in 1986, one of the industry's best for a Teamster union carrier.

Despite lower quarterly operating earnings, NW's net income was virtually unchanged at $2.2 million.

Mr. McMorris said the company also began to see a stabilization in trucking freight rates during the quarter after nearly a year of growing discounts.

For 1988, NW remains concerned about projected shortfalls in the nation's economic growth. Mr. McMorris said his company has hedged some of its equipment purchases until it can better evaluate the economic climate.