Johnson defeated Brickyard 400 pole-winner Kevin Harvick by 0.696 seconds to win the 40-lap race, his first in the series
that pits drivers from different circuits in identical stock
The top five drivers are from the NASCAR Winston Cup Series. Sixth-place Scott Sharp was the highest-finishing IRL driver in the 12-car field.
Two-time Indy 500 winner Helio Castroneves led the first 20 laps until the only yellow flag was flown for debris.
Busch was able to clinch the series title, becoming the seventh rookie in IROC history to win the championship.
Busch came in with an eight-point lead over defending Truck series champion Mike Bliss and won by 11.