The math behind this calculator (click to expand)
The financed amount is (vehicle_price - trade_in - down_payment) + sales_tax, where sales tax is calculated on the price minus trade-in value (in most states; a few like California tax the full price without the trade-in offset). Monthly payment uses the standard amortization formula M = P * [r(1+r)^n] / [(1+r)^n - 1], where r is the monthly interest rate (annual rate / 12) and n is the total months.
Each month: interest = balance * r, principal = monthly payment - interest, balance_next = balance - principal. The amortization table runs the iteration to zero balance. Total interest paid is the sum of monthly interest. The financed sales tax piece is what trips up most online calculators that treat the loan as price - down_payment, ignoring tax entirely; on a 7% sales tax state, that omission undercounts the loan by 7% of (price minus trade-in).
Implementation by Michael.
How Loan Term Length Reshapes Your Total Cost
The monthly payment difference between a 48-month and 72-month auto loan looks appealing on paper. In an illustrative scenario with a $32,750 SUV, $5,000 down, and 6.9% APR, the 48-month payment is about $663 versus $486 on 72 months - a $177/month spread. But that lower payment costs you: total interest jumps from $4,239 on the shorter term to $7,274 on the longer one, a $3,035 penalty for stretching payments. Replace 6.9% with the APR in an actual lender quote.
The 60-month term often hits the practical sweet spot. On that same $27,750 loan at 6.9%, the monthly payment lands around $548 with $5,130 in total interest. You avoid the steepest interest penalty of the 72- and 84-month options while keeping the payment roughly $115 lower than the aggressive 48-month schedule.
The Depreciation Trap: When You Owe More Than the Car Is Worth
A new vehicle loses roughly 20% of its value in year one and another 15% in year two. A $32,750 car is worth approximately $26,200 after 12 months and $22,270 after 24 months. If you financed $29,484 (after $5,000 down plus 6.25% sales tax) at 6.9% over 72 months, your remaining balance after 12 months is still about $25,680 - close to the car's depreciated value. After 24 months on a 72-month loan, you owe roughly $21,740 against a car worth $22,270 - barely above water.
On an 84-month loan the problem worsens. You can remain underwater for 3-4 years, which matters if the car is totaled, stolen, or you need to sell. Gap insurance covers the difference between what you owe and what insurance pays, but it is an added cost ($20-40/month) that only exists because the loan structure created the problem.
New vs. Used Rate Spreads
Auto-loan pricing varies by credit profile, vehicle, term, lender, and whether the car is new or used. On a $22,500 vehicle financed over 60 months, moving from an illustrative 7% APR to 9% adds about $1,340 in total interest. Compare written quotes using the same amount and term, including any origination fees or dealer incentives, before judging one offer by its monthly payment.
Down Payment Thresholds That Actually Matter
Most lenders do not have strict LTV (loan-to-value) cutoffs like mortgages, but putting at least 10% down on a new car or 10-20% on a used car accomplishes two things: it reduces your likelihood of being underwater from day one, and it can unlock lower interest rates from certain lenders. On a $32,750 new car, a $3,275 down payment (10%) versus a $6,550 payment (20%) changes the financed amount from $31,325 to $27,851 (assuming 6.25% tax on the net amount). At 6.9% over 60 months, that is roughly a $68/month difference and about $870 in interest savings.
Trade-in value functions identically to a down payment in the math - it reduces the amount financed. However, in most states, trade-in value also reduces the taxable purchase price. If you trade in a car worth $8,000 on a $32,750 purchase in a state with 7% sales tax, you save $560 in tax compared to selling privately and making an equivalent cash down payment. Factor that tax benefit against the typically higher proceeds from a private sale.
The Real Cost of Dealer Add-Ons
Extended warranties, paint protection, fabric coating, and VIN etching are high-margin dealer products often rolled into the loan. A $2,500 extended warranty financed at 6.9% over 60 months adds $49/month and $440 in interest - the true cost is $2,940, not $2,500. If you want the warranty, negotiate its price separately and pay cash if possible. The same principle applies to gap insurance: a dealer might charge $800 while your auto insurer offers it for $20-40/year (roughly $100-200 for the high-risk period).
Refinancing: When the Numbers Work
Auto loan refinancing makes sense when rates have dropped or your credit score has improved since the original loan. Moving a $24,000 remaining balance from 8.5% to 5.9% with 36 months left saves roughly $870 in interest. Most refinance lenders charge no fees (unlike mortgages), so the break-even is immediate. The catch: if you extend the term during refinancing (e.g., resetting from 36 remaining months to 60 new months), you will likely pay more total interest despite the lower rate. Refinance into an equal or shorter term to capture the full benefit.
What might change in the next 24 months
Auto-loan rates respond to broader interest rates, lender funding costs, credit risk, vehicle age, term, and market competition. Published averages are useful context but may not resemble a specific borrower's offer. Use the APR from a written quote in the calculator and compare it with at least one quote outside the dealership. For the longer historical context, see the history of auto-loan rates.
New-vehicle MSRPs continued to climb through 2025, with the average transaction price for a new vehicle near $48,000 (Cox Automotive). Loan term mix shifted accordingly: the share of loans at 84 months has grown to roughly 1 in 7 new-car loans, the share at 72+ months to about 60% (Experian State of the Automotive Finance Market). Long terms remain the lever buyers reach for, and the underwater-window risk in the new section above is the practical consequence.
Electric vehicle financing has its own quirks. Federal EV tax credit rules under IRA Section 30D continue to apply ($7,500 new, up to $4,000 used), but credit eligibility depends on battery sourcing rules that change annually. Some buyers transfer the credit to the dealer at point of sale to reduce financed amount immediately rather than waiting for the tax return. If you're buying an EV, run two scenarios: the loan with full price financed against the rebate received later, and the loan with the credit applied at sale. The interest savings on the second path is meaningful.