Rahal became the seventh Verizon IndyCar Series driver to win in the first seven races.
Rahal's father won the Detroit Grand Prix in 1992, the first year the race was run on Belle Isles after it was previously run on a street course through downtown Detroit.
Rahal made a last-lap pass of James Hinchcliffe for his previous win past year, when he beat Hinchcliffe by eight-thousandths of a second in the closest finish for an IndyCar race at Texas.
Honda's have been very fast in Chevrolet's backyard, sweeping the podium on Saturday and also getting the top three qualifying spots with James Hinchcliffe starting third.
Josef Newgarden, driving for Team Penske, was fourth in the only vehicle with a Chevy engine among the top seven in the 22-car field. Takuma Sato kept the momentum from his historic Indianapolis 500 win going, winning the Verizon P1 Award and pole position for the afternoon's second race. The second race of the Chevrolet Detroit Grand Prix is scheduled for Sunday.
All Offers and Promotions posted in this article excludes NSW residents. Castroneves' lap of 1 minute, 15.1511 seconds (112.573 mph) in the second practice put the driver of the No. 3 Hitachi Team Penske Chevrolet atop the combined speed chart for the day.
Despite dealing with an ankle injury after his wild crash in the Indianapolis 500, Scott Dixon battled past the pain barrier to take second place, moving into the lead of the drivers' championship in the process. Charlie Kimball couldn't avoid a slowing Conor Daly on the backstretch, and the contact that followed broke a piece of Kimball's auto.
Rahal's excitement was tempered by the fact his wife, drag racer Courtney Force, escaped without injury in a qualifying run Friday in Epping, N.H. Soon after leaving the starting line, her auto exploded after a fuel line burst.
Rahal went on to win that race. "But she seems to be OK".
Sunday's IndyCar schedule will follow a similar path here at the Belle Isle circuit. Mikhail Aleshin finished sixth and Helio Castroneves finished seventh in Saturday's opening race.
"I'm over the frustration already", he said. The auto split in half on impact with the inside wall and somehow the four-time series champion escaped with only a small foot fracture.
The race featured a total of six lead changes among five different drivers.
The open-wheel series shifts to Texas Motor Speedway, where Rahal is the defending champion, for a race under the lights Saturday night.