I won't suggest you to buy car using your 3.1 bitcoin because car is a depreciating investment. The value of your car will decrease over time. I respect that fact that, car is not always looked as an investment. It is mainly for commuting. But it is also important to look at your profits from bitcoin. So instead of buying a car for personal use, buy a car with commercial registration and rent it out to your local taxi service like Uber or Ola. With 3.1 bitcoin, I can buy a nice sedan car in my country and can rent for higher value taxi services. I think that will be good idea because you will earn some decent amount of money from it which will help you to maintain your project for bitcoin for longer term. Wish you all the best!