John Watson drove his McLaren racer to an easy victory in the inaugural Detroit Grand Prix yesterday, vaulting into the lead in the world championship point standings.

Watson, 36, took the lead on the 42nd of 62 laps around the 2.5-mile course that threaded around the downtown Renaissance Center. He stayed there, gaining his second triumph of the season and the fourth of a Formula 1 career that began in 1973.

The race was scheduled to cover 75 laps or two hours and was stopped by the time limit, with Watson averaging 78.2 mph on the slow circuit.

Eddie Cheever was second in a Ligier, 15.726 seconds behind Watson for the best finish of his young career.

Watson now has 26 points for the season, with Pironi second at 20.

Bobby Allison, who benefited from a friendly push from a fellow driver when he ran out of gas, won the rain-delayed Pocono 500-mile NASCAR race at Mount Pocono, Pa., for his third Grand National victory of the season.

Dry tanks affected both Allison and Richard Petty while they were battling for the lead in the late stages of the 200-lap race. Petty ran out of gas on the 155th lap and was pushed into pit row by his son Kyle.

Allison, who pitted with Petty for fuel on the 115th lap, lasted eight more circuits around the 2 1/2-mile track. He was around the third turn on the 163rd lap when he ran out of fuel and was pushed into the pits by Dave Marcis.

The race, constantly plagued by rain, was slowed in the 196th lap by a downpour and Allison drove comfortably, unchallenged, under the yellow caution for the remaining four laps . . .

At Mosport, Ontario, Al Unser Jr. scored his second consecutive victory in taking the rain-shortened $128,000 Can-Am race.