No, Tom didn't know who was driving the "death car." Because the yellow car belonged to Gatsby, he assumed Gatsby was the driver. "The God damned coward!" he whimpered. "He didn't even stop his car."
This incident only heightened Tom's dislike of and suspicions about Gatsby and his past. Earlier that afternoon, Gatsby and Tom had a confrontation regarding Daisy's future and with whom she would be spending it.
"You don't understand," said Gatsby, with a touch of panic. "You're not going to take care of her any more." "I'm not?" Tom opened his eyes wide and laughted. He could afford to control himself now. "Why's that?" "Daisy's leaving you." "Nonsense...I've made a little investigation into your affairs-and I'll carry it further tomorrow."
Tom was angry and threatened by Gatsby's presence, by Daisy's betrayal in paying attention to him, by his powerlessness to keep Daisy away from Gatsby, and by his inability to engineer his life so as to be able to spend time with Myrtle whenever he wanted without outside complications. All these conflicting emotions and situations came together along with the speeding car that hit and killed Myrtle. He was too grief-stricken, jealous, and frustrated to think that anyone other than Gatsby had been driving his car that night.