Keke Rosberg of Finland, overcoming an unscheduled stop for fresh tires 14 laps from the end, drove a Honda-powered Williams through downtown Detroit to a victory in that city's Formula 1 Grand Prix.

It was the fourth Formula 1 victory for Rosberg, 36.

He took the lead on the eighth of 63 laps around the 2.5-mile, 20-turn circuit snaking around the high-rise Renaissance Center. He stayed there the rest of the way, moving away to leads of more than 30 seconds halfway through the 160-mile event.

However, he was slowing late in the race with worn tires and, with his margin over the second-place Ferrari of Stefan Johansson of Sweden narrowed to 24.892 seconds after 49 laps, Rosberg drove into the pits for new tires.

The change took only 10 seconds and he came out just under two seconds ahead of Johansson. Johansson never was able to move closer and spun two laps from the end. He finished 57.549 seconds behind Rosberg.

Michele Alboreto finished third, adding to his point lead in the world championship standings . . .

The Mercury Capri of Wally Dallenbach Jr. was disqualified for being 25 pounds underweight and Elliott Forbes-Robinson, driving a Buick Somerset Regal, was declared the winner more than four hours after the Detroit round of the Sports Car Club of America Bendix Brake Trans-Am.

Capris, which have been dominating the series, must weigh at least 2,600 pounds without driver at the postrace weigh-in. Bob Anderson, vice president of the SCCA and chief steward of the Trans-Am series, said Dallenbach's car was weighed eight times over a "lengthy" period and each time came in under the minimum weight.

Forbes-Robinson was 23.9 seconds behind Dallenbach, who led the entire 100-mile race.