How Auto Loan Payments Are Calculated
An auto loan payment is calculated using the same amortization formula as a mortgage. The lender takes the amount you finance (vehicle price minus your down payment), applies the annual interest rate on a monthly basis, and spreads the total across the number of months in your loan term. The result is a fixed monthly payment that covers both principal and interest.
Like a mortgage, you pay proportionally more interest in the early months. As the principal balance decreases, more of each payment goes toward paying off the actual loan. This is why making extra payments early in the loan can save a meaningful amount in total interest.
New Car vs. Used Car Financing
Interest rates on used car loans are typically 1–3% higher than new car loans. This is because used vehicles are considered higher risk by lenders — they're harder to value, have more mechanical uncertainty, and depreciate unpredictably. However, the lower purchase price of a used car often means lower total interest costs despite the higher rate.
New cars typically lose 20–30% of their value in the first two years. If you finance with a long-term loan (72+ months) and a small down payment, you may owe more than the car is worth for much of the loan — a situation called being "upside down" or "underwater." This is risky because if the car is totaled or you need to sell, you'd still owe money after the sale.
How Your Down Payment Affects the Loan
A larger down payment reduces both your monthly payment and total interest cost. Putting 20% down is ideal because it helps you avoid negative equity from day one. Even if you can't put 20% down, every dollar you add to the down payment saves money over the life of the loan. Trade-in value from your current vehicle can also serve as a down payment.
The True Cost of Longer Loan Terms
Stretching a loan from 48 to 72 months drops your monthly payment, but dramatically increases total interest. For example, financing $30,000 at 5.9% for 48 months costs about $3,730 in interest. The same loan over 72 months costs about $5,700 in interest — over $2,000 more. Financial experts generally recommend keeping auto loans to 60 months or fewer.