Who does tom beleave killed myetail? What really happened?

In the valley of ashes, Nick, Jordan and Tom find that someone has been struck and killed by an automobile. The young Greek, Michaelis, who runs the coffee house next to Wilson's garage, tells them that the victim was Myrtle Wilson. She ran out into the road during a fight with her husband; there, she was struck by an opulent yellow car. Nick realizes that the fatal car must have been Gatsby's Rolls-Royce. Tom presumes that Gatsby was the driver.