Most auto-loan calculators tell you the monthly payment. They don't tell you what you're giving up by spending the money on a car instead of investing it. This one does both.
Down payment + monthly amounts contributed to a low-cost index fund over the same 5-year term.
A $40,000 car at 7% APR over 5 years costs you $46,771 total - including $6,771 in interest. The same money invested would grow to $56,705.
FinNudge's Cashflow Monitor agent projects your end-of-month balance daily - so you know before you sign whether a $600/mo car payment fits without breaking your budget.
Try FinNudge free →No credit card · Spark tier is freeThe monthly payment uses standard auto-loan amortization: P × [r(1+r)n] / [(1+r)n − 1], where P is the loan principal (price minus down), r is the monthly interest rate, and n is the number of months. Total paid is monthly × n plus the down payment. Total interest is what you paid minus the loan principal.
The opportunity-cost column uses future-value math at 7% annual return compounded monthly. The down payment is treated as a lump sum invested at term start; the monthly payment is treated as a series of contributions. Sum the two and you get what the money would have become if you'd invested it instead of buying the car.
Both calculations are pre-tax. We don't model car depreciation, registration, insurance, maintenance, or the value of actually owning a working car - those are real costs and benefits but they vary too much to baseline. The calculator surfaces the pure financial cost of the loan; you decide what the car is worth to you in non-financial terms.
7-year (84-month) auto loans now make up roughly 35% of new-car financing - up from less than 10% a decade ago. The pitch is always the same: lower monthly payment. The reality is that a longer term means more interest, more years you're underwater on the loan (owing more than the car is worth), and a higher chance that you trade or sell before the loan is done - rolling negative equity into your next car loan.
Play with the term picker above. A $35K car at 8% APR costs roughly $5,500 in interest over 48 months - and roughly $10,200 in interest over 84. The monthly payment drops about $260, but you pay almost double the interest to get there.
Because that is the actual trade-off. The dollars going into a car payment are dollars not going into an index fund, retirement account, or down payment savings. The opportunity-cost number is what those dollars would have grown to if invested at the long-run real return of the US stock market (~7%).
7% is the long-run inflation-adjusted return of the US stock market and the standard assumption financial planners use for retirement projections. Specific 5-year stretches have been higher or lower. Past performance does not guarantee future results.
No. Most cars depreciate roughly 50% over the first 5 years. The calculator shows the total paid to the lender; if you wanted a fair head-to-head you would subtract estimated resale value at end of term. The investment side is also pre-tax; capital gains taxes would reduce the realized return. We keep both columns simple and let you mentally adjust.
Use the rate from your pre-approved lender quote or your dealer offer. If you do not have one yet, current new-car average APR is roughly 7-8% as of 2026; used-car average is roughly 11-12%. Your credit score moves this materially - excellent credit can be 2-4 percentage points below average.
A regular auto-loan calculator tells you what you owe. This one tells you what you give up. The monthly payment math is identical. The opportunity-cost column is the addition.
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