01Why a longer term feels cheaper and costs more
On a $33,000 loan at 6%, moving from 60 to 84 months lowers the calculated payment from $637.98 to $482.08 — about $155.90 less per month — while adding about $2,216 of interest. The lower payment is real, but so are the extra years of financing and exposure to negative equity.
Vehicle value and loan principal follow different curves. On that 60-month example, about 36.45% of principal is repaid after two years. The calculator applies the selected New or Used planning curve to the actual dated ledger; it does not assume that every vehicle retains 58% in year three.
Edmunds’ Q3 2026 report found 29.1% of new-vehicle purchases with an outstanding-loan trade-in were underwater, with average negative equity of $7,107.
- The crossover month is the first modeled payment where vehicle value is at least the remaining balance. It is an estimate, not an appraisal.
- Rolling negative equity into a new loan increases amount financed and makes that portion nonqualifying principal for the federal interest deduction.
- Compare total interest and equity as well as payment. A longer term is not automatically wrong, but it is not a free discount.
02What drives the rate you are offered
Credit history, term, loan-to-value, vehicle, lender and market conditions all matter. Experian’s Q2 2026 averages run from 4.41% for superprime new-car buyers to 16.11% for deep subprime; used-car bands run from 6.29% to 21.62%. These are observed averages, not live offers.
On a $33,000, 60-month loan, the calculator’s 6.15% prime average produces about $5,417 of interest, while its 9.71% nonprime average produces about $8,787 — a difference of roughly $3,370. Use Exact APR for a written approval and keep the tier average visible as a benchmark.
03Vehicle tax is not one national sales-tax formula
Many states reduce the taxable base by a qualifying dealer trade-in, but the details matter. California, Virginia and DC do not get the generic full trade credit in this model; Michigan has a date-dependent cap; Oklahoma splits excise and sales-tax bases; Kentucky and Maryland do allow qualifying trade treatment. Local taxes and agency fees can be additional.
Special motor-vehicle charges need special arithmetic: Delaware has a 5.25% document fee with per-$100/fraction rounding, Iowa uses $10 plus 5%, South Carolina caps its IMF at $500, DC uses weight and MPG/EV bands, and Washington can add a luxury tax. Oregon’s dealer privilege tax and Hawaii’s business GET are not automatically presented as buyer-paid sales tax.
Use the state estimate as a starting point, then enter a verified combined override when the buyer’s quote, local tax or transaction-specific base differs.
04The 2025–2028 qualified vehicle interest deduction
The federal deduction is above the line, so itemizing is not required. It covers up to $10,000 of qualified passenger vehicle loan interest per return per tax year from 2025 through 2028. The statutory conditions include original use with the taxpayer, personal use, a qualifying road vehicle below 14,000 pounds GVWR, U.S. final assembly, a post-2024 first-lien loan and VIN reporting. Confirm assembly with the vehicle label or NHTSA decoder rather than the brand alone.
MAGI reduces the deduction by $200 for each $1,000 or part above $100,000, or $200,000 on a joint return. The $150,000/$250,000 endpoints assume a full $10,000 is otherwise available; a smaller deduction can be exhausted earlier. Married filing separately is not categorically excluded by the final rules. The calculator uses interest paid in the selected tax year, not the first 12 loan months.
T.D. 10054 also requires principal allocation. Down payment is applied against negative equity first, and only the qualifying share of each interest payment counts. Estimated tax savings equal deduction multiplied by the editable marginal-rate assumption; the deduction itself does not reduce the lender’s interest charge.
A deduction reduces taxable income, not the interest bill. At a hypothetical 22% marginal rate, a $3,000 deduction saves $660 — not $3,000.