One could argue that Tom Buchanan had no idea that Daisy was driving Gatsby's car and was the person responsible for Myrtle's death. Shortly after Tom discovered that Myrtle Wilson was run over by a big yellow car, Nick mentions that Tom slammed on the gas towards home and continued to mumble to himself,
The God Damn coward! . . . He didn’t even stop his car. (Fitzgerald, 151)
Tom's comment clearly indicates that he believes Gatsby is responsible for killing Myrtle. It is also important to note that moments before the accident, Tom told Gatsby to take Daisy home after exposing him as a greedy bootlegger. Tom also despises Gatsby and is a completely selfish individual who lacks perspective and believes that Gatsby was motivated to kill Myrtle as payback for exposing him as a bootlegger. In the final chapter of the novel, Nick runs into Tom Buchanan on Fifth Avenue, and Tom once again reveals that he believes Gatsby ran over Myrtle by telling Nick,
That fellow had it coming to him. He threw dust into your eyes just like he did in Daisy’s but he was a tough one. He ran over Myrtle like you’d run over a dog and never even stopped his car. (Fitzgerald, 191)
In addition to Tom's comments, it is highly unlikely that Daisy would confess her involvement in the accident to her husband. As a shallow, immoral individual, Daisy seems comfortable allowing Gatsby to take the blame for what she did.