Nick learns from Jordan Baker the details of Tom's behavior in Santa Barbara after his and Daisy's honeymoon. She tells Nick that Tom was involved in a car accident that made the newspapers. He was with another woman, a maid at the Santa Barbara Hotel. Her arm was broken. This shows Tom's infidelity began early in his marriage and continued. Daisy was aware of it (everyone who read the newspaper was aware of it), but she remained in her marriage and gave birth to their daughter several months later.
Jordan remembers seeing Daisy in Santa Barbara as a new bride madly in love with Tom after their honeymoon in the South Seas. Daisy would "sit in the sand with his head in her hand by the hour, rubbing her fingers over his eyes and looking at him with unfathomable delight." But Tom betrays this devotion by having an affair with a hotel chambermaid. There's a scandal when he has a car accident one night with the chambermaid and rips "a front wheel off his car." The chambermaid appears in the news along with Tom, because she breaks her arm in the crash.
The story comes down to us from Jordan and then through Nick, but we have reason to believe it is true because Daisy, the first night she has Nick over to her home, alludes to being made miserable in her marriage and the fact that Tom is currently having an affair with a lower-class woman. Tom's earlier love affair, so soon after his marriage, lends credence to Nick's conclusion that Tom and Daisy are careless people who damage people and things and then move on, retreating into their vast fortune. This Santa Barbara car accident involving a lover foreshadows the later, more tragic accident with Myrtle, and also is more or less a repetition of the accident with Owl Eyes leaving Gatsby's.
Further, despite Tom's steadfast conviction that he and Gatsby have nothing in common, the circumstances of Tom's extravagant wedding, unlike anything Louisville had ever seen, the $350,000 strand of pearls he gives Daisy (expensive now, but stupendously expensive then) and the three-month honeymoon, sound not all that different from Gatsby's way of trying to woo Daisy with his wealth.