The 20/4/10 rule is sensible because it is an easy rule to stick to when car shopping. The name is easy to remember and is easy to calculate. While this is not the most important thing to consider when purchasing a car, it is a good place to start when beginning to shop for cars. It is important to consider what really works best for your lifestyle, financial history and future, income, other costs, and other factors that will determine what vehicle you purchase and what auto loan you take out.