Why does Tom allow Daisy to ride alone in the car with Gatsby? What has changed ?

Tom allowed Daisy to ride home with Gatsby because he no longer felt threatened. He knew she would never leave him for Gatsby.

“please, Tom! I can’t stand this any more.”
Her frightened eyes told that whatever intentions, whatever courage, she had had, were definitely gone.
“You two start on home, Daisy,” said Tom. “In Mr. Gatsby’s car.”
She looked at Tom, alarmed now, but he insisted with magnanimous scorn.
“Go on. He won’t annoy you. I think he realizes that his presumptuous little flirtation is over.”