What is significance of Tom making the choice in telling Wilson that it was Gatsby's car?

I read the book so if you are going to give me crap about it don't. I just need to be reminded because I had it in my head and then I lost it. So please if you have read the book could you please help me by giving me your opinion of what the significance of Tom's choice in telling Wilson that the yellow car was Gatsby's. Thanks. :) :) :)