Upvote:19
Having a car payment (or not) is highly unlikely to be material to your visa application. They're really looking for two things: Do you have a stable income that lets you afford the trip to the UK? Do you have strong ties to your home country so you leave after the trip is over? Having a partly paid car at home is mildly useful, but it's not going to make or break your visa.
The only reason you might want to mention the car is if the 20% down payment makes your October cash flow deeply negative. If so, you should note that this was a one-off and the following monthly payments are affordable for you.
This epic answer goes into much more detail about this general topic: Should I submit bank statements when applying for a UK Visa? What do they say about me?