An alternative to the $20\%$ rule is to evaluate your budget, determine a monthly payment that you can reasonably afford, and then incorporate that information with the maturity and interest rate of a possible loan to determine the value of the potential loan. When this value is added to a saved amount of a down payment, you know the total amount that you can reasonably afford to spend on a new car.

To review this process, consider the following case:

Larry's Car$-$Buying Decisions

Larry, who lives in San Francisco, is trying to decide between the following car models:

$\backslash$table$[[$Brand and Model,Cost$],[$Ford Fiesta,$$13,580$