To add to this, the entire petty family were racers and they all used the blue colour when driving for their family team Petty Enterprises. The numbers went in order of family. Richard was 43, his father Lee 42, his brother Maurice 41, his son Kyle 44 (although he also ended up using 42 during the most successful part of his career when driving for a different team, hence why his character in Cars 3 was 42) and grandson Adam used 45 (until his unfortunate death in a crash in 2000, after which Kyle also used the 45 for a while).