When you buy a car it will always have the fuel tank filled up to 80%. Buy a car, pick a race and in the Car Setting menu before you start the race you sell your fuel down to the minimum 15%, which earns you 195 credits, then exit back to the Grand Prix menu and enter the garage, you then sell your car for the original price while keeping the 195 credits you earned from selling the fuel. When you buy the car again it will have 80% fuel again, you can then repeat selling the fuel and the car and buying the car again as many times as you need. It's a slow process to earn a lot of credits but if you are close to what you need to buy a better car and you don't feel like going through an entire Grand Prix to earn those few extra credits then this can be a nice little shortcut.