What is the ruling on returning an usurious loan to the bank, and how does one repent for taking such a loan?

Purchasing a car with riba (usury/interest) is a grave sin and one of the major transgressions. If you sincerely repent to Allah, we hope for your forgiveness. If you are able to pay back the principal amount of the debt without the usurious profit, then do so. Otherwise, there is no blame upon you for paying the full amount if you are compelled to do so.