We find the answer to your question in the final chapter of this incredible novel. Nick meets Tom Buchanan again after Gatsby's death, and asks him outright what he said to Wilson on the afternoon of the murder. The silence that Nick is initially greeted with confirms his suspicions, and then Tom confesses:
"I told him the truth," he said. "He came to the door while we were getting ready to leave, and when I sent down word that we weren't in he tried to force his way upstairs. He was crazy enough to kill me if I hadn't told him who owned the car. His hand was on a revolver in his pocket every minute he was in the house--" He broke of defiantly. "What if I did tell him? 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."
Note how Tom reveals his true colours in his speech. Although he likes to come across as so strong and arrogant, clearly feeling himself superior to other people, he shows himself to be a coward at this stage and also he confesses that he used Wilson to get his own revenge on Gatsby for what he has done to his marriage and for his own sense of pride at being cuckolded. This only serves to highlight Nick's impression of the Buchanans as the kind of people that break everything they play with, leaving it and going on to their next "toy."