Why does Tom lie and say he bought Gatsby's yellow car a week before?
This is before when they are going into the city and Tom, Jordan, and Nick are in Gatsby's car. While Daisy and Gatsby are in a coupe behind them! Please and thank you! :)
Remember the start of chapter 2 when Tom takes Nick to the city and they stop at Wilson’s garage? It is obvious that Tom stops here often to see Myrtle and tell her to meet him in the city. Fitzgerald infers this for us when he has Wilson ask Tom about the car he (Tom) wants to sell. From this conversation, we are supposed to understand that Tom uses the excuse of having a car to sell as his reason for frequently stopping at Wilson’s garage. So, when he drives Gatsby’s car to the city, he arrogantly tells Wilson he just bought the car (to prove that he did, indeed, plan to sell it all along) . This statement will, of course, later come back to haunt him.
In the Great Gatsby,Tom is someone extremely rich and looks down upon others.For him,poor people doesn't have any importance.So as George Wilson is poor, he looks down upon him.
In chapter2 when Nick accompanied Tom to the Ash Valley, at that time as well he was asking George whether he is interested in buying his car;although he knows that Wilson is poor and won't be able to buy the car,he was pulling his leg.It is the same case that happened with the car of Gatsby when Tom drove it to wilson's garrage,he was pulling his leg and at the same time he was proving his arrogance.