DETROIT (AP) _ Ford Motor Co. leads the U.S. Big Three automakers in productivity at its plants while General Motors Corp. continues to trail the pack, according to a study released today.

The Harbour Report, produced by former Chrysler Corp. manufacturing executive James Harbour, said Ford was the low-cost producer of cars and trucks, requiring just 3.01 workers to assemble a vehicle compared with 3.76 for Chrysler and 4.55 for GM.

Ford's plant in Kansas City that makes the Tempo and Topaz compacts was the best of all car assembly plants, requiring 2.37 workers per car. Ford also had the No. 2 through No. 5 best plants, tying for fifth with Chrysler's Bramalea, Ontario, plant, that is building Chrysler's new midsize cars.

Among truck assembly plants, Ford also had a sweep with its F-truck plant requiring just 2.31 workers per vehicle to assemble.

Chrysler's new Jefferson North plant in Detroit, home of the Jeep Grand Cherokee sport utility truck, was rated as the most-improved plant. It was compared with the aged Jefferson Avenue plant, razed to make room for the new plant.

Chrysler's Jeep plant in Toledo, Ohio, was the second most-improved plant, boosting productivity 25 percent since 1989.

GM had the third and fourth most-improved plants, its Oshawa, Ontario No. 1 plant that makes the Chevrolet Lumina and its Oshawa Truck plant which makes its C-K trucks, were up 23 percent and 22 percent respectively.

Chrysler's Belvidere, Ill., and GM's Fairfax, Kan., plant were tied for fifth with 21 percent productivity improvement. Belvidere makes the Dodge Dynasty, but will soon be retooled for a new generation of Chrysler small cars. Fairfax makes the Pontiac Grand Prix.

All of the Big Three required fewer workers to make vehicles than during the last Harbour study in 1989. Chrysler had the sharpest drop, from 4.58 workers per vehicle. GM, whose ratio of workers to vehicles built was practically stable from 1979 to 1989, dropped from 4.88, while Ford improved from 3.25 workers per vehicle.