After Adam Petty's throttle stuck wide open in a practice session he turned his car to the left and it spun out. He got out of the car shaken up and NASCAR found out that his car had a faulty throttle. After this, he got a new car for the race and would wind up finishing 6th in the final Busch Series championship standings, after winning 1 race.
Starting in 2001, Richard and Kyle move Adam up full time in Winston Cup racing. In Adam's first full season, Sprint signed on as Full time sponsor for the 45 Dodge. In the 2001 Daytona 500, Adam was drafting with Sterling Marlin on the last lap when Dale Earnhardt hit the wall in turn 4, Adam ended up finishing the race 4th and was in tears during his post-race interview. Adam would end up winning 1 race, at Pocono where he held off Ricky Rudd for the win. He finished 16th in final point standings with 1 win, 0 poles, 3 top 5s, 12 top 10s, and 4 DNFs, and was was runner up for Rookie of the Year.
Adam would start the 2002 season by winning the pole for the Daytona 500 but would be involved in a wreck on lap 148, finishing 32nd. He would shock everyone by winning at Rockingham over Matt Kenseth by spinning him coming to the Caution flag, he would finish 6th at Las Vegas and 9th at Atlanta, Going into Rockingham he finished 2nd giving him 2nd in points, but at Bristol he would finish 13th after hitting the wall after battling Kevin Harvick for 10th. At Texas, Adam led 264 laps and finished 3rd, giving him enough Momentum to win at Martinsville after leading 379 laps. Adam then blew an engine on lap 4 and finished 43rd at Talladega, then a 32nd place finish at Califorina and a 40th at Richmond sent him to 7th in points, but a runner-up finish in the Coca-Cola 600 helped him, but he slunk back into a slump for 6 straight races with finishes of 38th, 36th, 21st, 43rd, 40th, 19th, until he finished 6th at New Hampshire which helped him back into the Top 10 in points. At Pocono, he was leading late until he blew a tire and the caution was thrown, he was able to rebound and finish 5th, going into Indy, he started second and finished 4th which he later said after the race "it feels good to finally have a run like this after so long". At Watkins Glen, Petty struggled most of the race but some how finished 10th, heading into Michigan he qualfied on the pole and finished in the same postion. At Richmond he finished 4th and at New Hampshire he finished 16th, but at Dover he dominated and finished 3rd, making him 13 points behind Sterling Marlin for the points lead. At Kansas, He finished 2nd after leading the 2nd most laps and gave him the points lead. At Talladega, Petty finished 3rd and led the most laps but was passed on the last lap by Stewart and Jr. At Lowe's however, Petty blew an Engine and was regulated to 42nd. At Martinsville he finished 12th and at Atlanta he finished 6th. At Rockingham he finished 19th and lost the points lead, At Phoenix he finished 10th but was unable to catch Tony Stewart even with a runner up finish at Homestead and lost the championship. He finished with 3 wins, 2 poles, 17 top 5s, 25 top 10s, 6 DNFs.
In 2003, following his 2nd place finish in the Championship standing, Stanley Tools signed a 2 year deal with him. He started by finishing 9th in the Daytona 500, then 19th at Rockingham and 23rd at Vegas. He would only win at Atlanta and would finish 2nd 4 times (Daytona, Darlington, Dover, Infineon) He would finish 7th in the final points standings with 1 win, 0 poles, 14 top 5s, 21 top 10s 2 DNFs.
2004, Sprint leaves his 45 Dodge Team and he started it by finishing 16th in the Daytona 500 followed by 3rd at Rockingham, 5th at Vegas, 17th Atlanta, He would then win at Darlington, followed by 2nd at Bristol, then a win at Texas, and Martinsville, which gave him a point lead which he would never lose until the Chase began. He would finish no worse than 23rd til the Chase began, winning the last race before the Chase giving him 3rd in points to start the Chase. He finished 3rd at New Hampshire, then won 2nd at Dover, followed by 7th at Talladega. He would then Dominate at Kansas leading 236 out of 267 laps. Followed by 10th at Lowe's and 2nd at Martinsville, and a win at Atlanta by passing Jimmie Johnson with 8 to go. At Phoenix he would finish 8th and 6th at Darlington he finished 2nd at Homestead, and won the Title by 2 points, *he was 1 point behind Kurt Busch going into Homestead and if he finished 2 places ahead he would win it*.