Why does Tom say, “That yellow car i was driving this afternoon wasn't mine---do you hear? I haven't seen it all afternoon?” What are his potential motives?

Tom desperately does not want to be connected with the car that killed Myrtle or his affair with Myrtle. He is quick to say that the car belongs to Gatsby. Tom wants Gatsby to take the fall for Myrtle's death and avoid any responsibility himself.