When to Use Annual Percentage Yield. Your car lender will often advertise a rate in terms of APR. This means the lender is not showing you the cost of compounding the interest. When you are seeking a new loan or debt, you should always calculate the annual percentage yield in order to determine a more accurate financing cost. Consider the following example. You have a $5,000 car loan with an interest rate of 2% per month over a 48 month period. APR = 12 months (.02 per month) = 2.4% APR.