2002 was Busch's breakout year in the Winston Cup Series, scoring four wins, 12 top fives and 20 top tens, with one pole, and finishing third in the final standings. He finished the season particularly strong, winning three out of the final five races and finishing third and sixth and leading many laps in the other two. He also collected
$5,105,394.
Busch had an up and down year in
2003, earning four wins, nine top fives and 14 top tens but finishing only 11th in the standings, although he collected US$5 million+ dollars again that year. It was a season marred by an ongoing feud with fellow driver
Jimmy Spencer. After some car-to-car bumping at a race at the
Michigan International Speedway, Jimmy Spencer reached into Busch's car, grabbed him, and punched Busch in the nose. Kurt first claimed that his car stalled out in front of Spencer's garage, though audio later revealed that he stopped the car, revved the engine, and threatened Spencer from his car. Spencer was forced to sit out the next week's race, and both drivers were fined and placed on probation for the rest of the year. The altercation harmed Busch's relationship with fans significantly. He has since attempted to repair this image through charitable donations and many public appearances where he interacts with fans.
In
2004, Busch won three races, two poles and the inaugrual NASCAR Nextel Cup Championship. He scored ten top five and 21 top ten finishes that season.
Midway through the 2005 season, Busch announced that he would be leaving Roush Racing at the end of the season and would replace Rusty Wallace in the #2 Miller Lite Dodge for Penske Racing South. Busch won three races during the 2005 season, along with nine top five and 18 top ten finishes in 3400 races. He finished 10th in the final standings despite missing the final two races after being prematurely released from his Roush Racing contract.