Frequently asked questions

The questions people ask most about US auto loan maths, and how CarLoanMath calculates each figure.

How much car can I afford?

No law sets this, only rules of thumb. The “20/4/10” rule means 20% down, a loan of four years or less, and total car costs around 10% of income. Edmunds suggests a payment of no more than about 15% of take-home pay for a new car (10% for used or a lease), with fuel and insurance on top. CarLoanMath turns a monthly budget into an amount financed by running the payment formula in reverse. At 7%, a $400 payment supports about $16,704 financed over 48 months or $20,201 over 60 months, before tax and fees. Insurance, fuel, repairs and your own budget can make the right number lower. Open the 48-month example ($16,704 at 7% is $400 a month). AI assistants connected to the MCP can ask the same question with the max_vehicle_price_for_budget tool.

Does a longer loan (60 vs 72 vs 84 months) cost more?

At the same rate, a longer term lowers the payment but raises total interest, and longer terms often carry higher rates too. Example, $30,000 at 7%: 60 months is $594.04 a month and $5,642.16 total interest; 72 months is $511.47 and $6,825.85; 84 months is $452.78 and $8,033.55. The CFPB notes that longer loans also make it more likely you will owe more than the car is worth. Open the 60-month example, then change the term to 72 or 84 months to compare.

Why is the dealer’s monthly payment different from my calculator?

Usually the inputs differ: the amount financed may include tax, fees, add-ons (for example a service contract) or negative equity, or the approved rate or term isn’t the one you assumed. CarLoanMath compounds the yearly rate you enter monthly, applies sales tax to the full vehicle price with no trade-in credit, and by default treats tax and fees as paid upfront, not financed. Lenders that accrue interest daily from the contract date, or a long gap before the first payment, can move the payment by a few dollars. Compare the Truth in Lending “Amount Financed,” APR, number of payments and payment amount line by line. No calculator can guarantee a cent-exact match without the contract. See the CFPB’s guide to the disclosure.

How does negative equity work when I trade in a car I still owe on?

Negative equity is your loan payoff minus the trade-in allowance. If you don’t pay that gap in cash, CarLoanMath adds it to the new loan (the “still owed” field) and you pay interest on it for the full new term. Example: $4,000 rolled into an 8%, 72-month loan adds $70.13 a month and $1,049.57 of interest. Positive equity (the trade-in is worth more than you owe) lowers the amount financed instead. Get the lender’s actual payoff amount, which can differ from your statement balance. The CFPB also warns to check that a dealer’s promise to “pay off” the old loan isn’t quietly financed into the new one. Open an example with $4,000 rolled in.

Should I pay cash or finance at a low APR?

Arithmetically, financing beats paying cash only if the cash you keep earns more after tax than the loan costs, and only if financing doesn’t cost you a cash-only discount or rebate. Compare the total cost of each path for the same car and price — in CarLoanMath, total cost is cash due at purchase plus all payments. A 0% loan has no interest, so total cost equals the price plus tax and fees (minus any trade-in, plus anything still owed). Whether to take on debt at all is a judgment no calculator can settle. Also check for prepayment limits and the insurance a lender requires.

Which is better: 0% APR or a cash rebate?

Compare totals on the same car and term. The 0% path costs the price (minus down payment) divided over the term — at 0%, CarLoanMath’s payment is simply the amount financed ÷ months. The rebate path is a smaller loan at your outside rate. Example, $40,000 over 60 months: 0% is $666.67 a month, $40,000 total; a $3,000 rebate plus a 6.5% loan on $37,000 is $723.95 a month, $43,436.85 total. Here 0% wins, and the break-even outside rate is about 3.11%. Enter the price after the rebate, not the rebate as a down payment, so total cost doesn’t treat the rebate as cash you paid. 0% offers often require top-tier credit and shorter terms; see the CFPB on qualifying. Tax treatment of rebates varies by state (see Are manufacturer rebates taxed?).

If I pay extra, does my monthly payment go down or does the loan get shorter?

On a standard simple-interest auto loan, extra principal shortens the loan and cuts total interest; the required payment normally stays the same. Some lenders apply extra money to future installments (“paid ahead”), so the bill may show a lower amount due. Paying only that lower amount undoes the benefit, so tell the lender to apply extras to principal. Illustration with CarLoanMath’s monthly compounding: $30,000 at 7% for 60 months plus $100 a month extra pays off in 50 months and saves about $972 of interest. The calculator does not have an extra-payment field, so this figure is a schedule run with the same formula, not a result the page will show.

Does a trade-in lower the sales tax?

In many states that tax vehicle sales, tax is charged on the price minus the trade-in allowance. Example: a $50,000 car with a $10,000 trade-in at 8% would be $3,200 tax with that credit, versus $4,000 without it. Some states give no credit, or only a partial one — California’s tax agency, for example, counts traded property in the taxable price. Rules change, and a private sale and a trade-in can be taxed differently, so check your state revenue or DMV site. CarLoanMath always applies the tax rate to the full vehicle price, with no trade-in credit, so that example shows $4,000 of tax. If your state taxes only the difference, adjust the tax rate so the tax line matches what you expect. Open the $50,000 example and look at the tax line.

How is the monthly payment calculated?

For a fixed-rate loan with monthly payments, payment = P × r ÷ (1 − (1 + r)^−n), where P is the amount financed, r is the yearly rate ÷ 12 and n is the number of months. At 0% it is simply P ÷ n. Amount financed is the price, minus down payment and trade-in, plus anything still owed on the trade-in — and plus tax and fees if you add them to the loan. Example: $30,000 at 7% for 60 months is $594.04 a month; at 0% it is $500.00. A useful check: $40,000 at 5% for 60 months is $754.85. Totals use the unrounded payment; figures are rounded to cents only at the end. Lenders that accrue interest daily can come out slightly different. Open the $40,000 at 5% example. See the CFPB on amortization.

How much should I put down, and what does it change?

A bigger down payment lowers the amount financed, the payment and total interest, and lowers your loan-to-value (LTV). The CFPB says a lower LTV may also get you a lower rate. Example, $35,000 at 7% for 60 months: $0 down is $693.04 a month and $6,582.52 interest; $7,000 (20%) down is $554.43 a month and $5,266.01 interest. No single “right” percentage exists; keep enough cash for emergencies. In CarLoanMath the down payment is capped at the vehicle price.

How much total interest will I pay, and what rate am I actually paying?

Total interest is all the payments minus the amount financed. Example: $30,000 at 7% for 72 months is $511.47 × 72 − $30,000 = $6,825.85. That figure assumes every payment is on time and on schedule; paying early on a simple-interest loan reduces it. CarLoanMath shows this total interest on the result card. To find the rate from a known payment, solve the same formula for r (a spreadsheet RATE function can do it). For an exact comparison, use the rate on the contract, not a guess. The Truth in Lending disclosure states the APR and the finance charge.

Why does so much of my early payment go to interest?

Interest is charged on the remaining balance, so early payments are interest-heavy and later ones mostly principal. Example: on $30,000 at 7% for 72 months, the first month’s interest is $175.00 of the $511.47 payment. The first bill can also include interest accrued since the contract date, and late payments add days of interest. CarLoanMath uses this same monthly split; AI assistants can see it year by year or month by month with the MCP’s amortization_schedule tool. Open this example.

If I pay my car off early, do I still owe all the interest?

With a simple-interest loan (the common type), you owe interest only up to the payoff date, so paying early saves interest. Ask the lender for a dated payoff quote. Your contract and state law decide whether a prepayment penalty applies; the CFPB says some states prohibit them. Check the Truth in Lending disclosure, which states whether you can prepay without penalty. Precomputed loans work differently (see What are precomputed or “Rule of 78” loans?). CarLoanMath’s total interest assumes every scheduled payment is made on time; it does not compute a payoff quote.

Do biweekly payments save money?

On a daily simple-interest loan, paying half the payment every 14 days can lower the average balance slightly. It also adds the equivalent of one extra monthly payment per year (26 half-payments), and that extra payment is most of the effect. Results depend on how your lender posts partial payments; some hold them until a full payment arrives, so confirm before relying on this. CarLoanMath models monthly payments only and does not have a biweekly mode, so it cannot show that saving.

What are precomputed or “Rule of 78” loans, and do they change the maths?

With precomputed interest, total interest is fixed at signing and spread across the payments. Extra payments don’t reduce the interest the same way, though paying off early may earn a partial refund of “unearned” interest. The CFPB calls this method uncommon; most auto loans use simple interest. Federal law (15 U.S.C. § 1615) bars Rule of 78 refund calculations on consumer loans longer than 61 months, and some states restrict precomputed loans further. Look for “precomputed,” “Rule of 78” or “simple interest” in the contract before assuming extra payments save money. CarLoanMath always uses the standard simple-interest amortization formula, not a precomputed or Rule of 78 schedule.

Is a longer term at a higher rate cheaper if I pay it like the shorter loan?

Usually not. If you pay the same amount each month, the loan with the higher rate charges more interest on the same balances. Example: $42,000 over 60 months at 8.49% is $861.49 a month and $9,689.51 interest. Paying that same $861.49 on a 72-month loan at 8.99% clears it in 61 months with about $10,461 interest, roughly $771 more. The only gain is flexibility: you can drop back to the lower required payment if money gets tight. CarLoanMath does not have an extra-payment field, so the 61-month figure is a schedule run with the same monthly formula; the 72-month example on the page shows the required payment of $756.86 if you don’t pay extra.

What’s the difference between APR and the interest rate?

The interest rate is the cost of borrowing the principal. The APR adds certain mandatory lender fees and states the total as a yearly rate, so it can be higher. The CFPB explains the difference. CarLoanMath’s rate field is labelled “Yearly rate, not APR” because it compounds the rate you type monthly and does not fold in fees; the MCP’s apr parameter is the same number. If you only know the APR, entering it gives a close, slightly cautious estimate. For an exact payment, use the rate the contract actually applies. Compare APR to APR, never APR to an interest rate.

Are manufacturer rebates taxed?

It depends on the state. Some states tax the vehicle price before a manufacturer rebate, so a $2,000 rebate on a $50,000 car can still leave tax due on $50,000. Dealer discounts are sometimes treated differently from manufacturer rebates. Washington’s Department of Revenue, for example, treats manufacturer rebates as part of the selling price and dealer rebates as tax-free discounts. Rules vary and change, so check with your own state’s revenue department. CarLoanMath does not apply rebate tax rules: it taxes the price you enter at the rate you enter, with no rebate adjustment. Enter the price you will actually be taxed on, and the tax rate you expect.

Should I roll sales tax and fees into the loan or pay them upfront?

Paying them upfront works like a bigger down payment: you borrow less and pay less interest. Rolling them in keeps cash on hand but adds to the payment and to interest. Example: financing $2,500 of tax and fees at 7% for 60 months adds $49.50 a month and $470.18 of interest. In CarLoanMath, Pay upfront (the default) keeps tax and fees in cash due at purchase; Add to loan finances them. Open a $30,000 example with $2,100 tax (7%) and $400 fees paid upfront, then switch to Add to loan to see the difference. Whether tax can be financed, and which fees exist, varies by state and lender.

What is a doc fee, is it capped, and how does it affect my loan?

A documentary (“doc”) fee is a dealer charge for paperwork, not a government fee. If it is financed, CarLoanMath adds it to the amount financed (under fees, with Add to loan) and it accrues interest like the price itself; if you Pay upfront, it goes into cash due at purchase instead. Caps vary by state. Iowa law caps it at $180. Indiana’s dealer division says it will not take enforcement action on fees at or below $261.72 as of July 1, 2026. Some states have no cap, so check your state’s rule and compare out-the-door totals. Enter the fee you are actually quoted; CarLoanMath does not apply state caps.

What does “total cost” include?

Cash due at purchase plus all the loan payments. That is the same as the price, plus tax and fees, plus interest, minus the trade-in value, plus anything still owed on the trade-in. A trade-in that is worth more than you owe lowers total cost; negative equity raises it by the gap (and by the interest on that gap). It does not include insurance, fuel, maintenance or future registration renewals.

Where are my numbers stored?

Your scenarios are saved only in this browser (local storage). There is no account and no sign-up. A share link carries just the loan inputs in its address, no name or email, so anyone you send it to sees the same numbers. Like any website, our server receives the page addresses you open.

Can I use CarLoanMath from an AI assistant?

Yes. CarLoanMath has an MCP server at https://carloanmath.com/mcp with the same maths as this page. See how to connect it to Claude, Cursor or ChatGPT.

Is this a loan offer?

No. These are estimates only, not a loan offer. Your lender’s numbers depend on your credit, the lender’s fees and your state’s rules.

Estimates only, not a loan offer.