A guide to the break-even math behind choosing whether to throw an extra $300 a month at your auto loan or drop it into a brokerage account.
Published: April 17, 2026
It's a common milestone: you have $400 extra per month. Should you funnel it into your auto loan principal to rid yourself of the monthly payment faster, or invest it to start compounding returns?
For most people, paying off debt feels psychologically liberating. But in the world of personal finance, peace of mind and mathematical optimization sometimes point in opposite directions.
Depending on when your car loan was originated, your interest rate dictates the right strategy. The key to the equation is understanding your Opportunity Risk Spread.
When you make an extra principle payment on a loan with a 6% interest rate, you are effectively earning a guaranteed, tax-free 6% return on investment. That's because you are successfully preventing the lender from charging you that 6%.
If you choose to invest that extra money instead, you must find an investment that yields more than 6% after taxes just to break even. This is known as an arbitrage opportunity.
Use this rule of thumb: Compare the APR of your car loan against the after-tax expected yield of your investment portfolio.
Scenario A: Low-Interest Rate (Under 4%)
Suppose you secured a promotional 2.9% auto loan for $25,000 over 60 months. If you invest $300 extra a month into the stock market (yielding an average of 8%), the math heavily favors investing. The spread between 8% gained and 2.9% lost is 5.1%. Paying off a 2.9% loan early is destroying wealth in the long run.
Scenario B: High-Interest Rate (Over 7%)
If poor credit or market conditions landed you an 8.5% auto loan, the spread reverses. A guaranteed 8.5% return from debt elimination is incredibly valuable. It is very difficult to reliably beat 8.5% in the stock market continuously without taking on massive risk. In this scenario, pay the car loan as fast as humanly possible.
Cars depreciate rapidly. In fact, a new car loses about 20% of its value in the first year alone. If you have a longer loan term (like 72 or 84 months), making minimum payments guarantees you will be "underwater" (owing more than the car is worth) for several years. Even if the interest rate is relatively low, some prefer to pay extra just to maintain positive equity and have the power to easily sell the vehicle without bringing cash to the closing table.
Mathematically, no. Since high-yield savings accounts and standard index funds typically yield substantially more than 3%, you will likely build more net worth by making minimum payments and investing the difference.
If your auto loan has an interest rate above 6-8%, paying it off provides a guaranteed, tax-free return that is hard to beat reliably through stock investing.