CarCostMath Methodology

What this page covers

• Financed amount = vehicle price + estimated sales tax - down payment, clamped at $0 so cash purchases and overpayments do not create negative loans.
• Loan payment uses standard amortization: principal × monthly rate × (1 + monthly rate)^term ÷ ((1 + monthly rate)^term - 1). At 0% APR, it falls back to principal ÷ term.
• Fuel or energy cost = annual miles ÷ MPG/MPGe × user-entered fuel or charging price ÷ 12. MPG/MPGe is guarded against zero to avoid divide-by-zero results.
• Depreciation = price × five-year depreciation percentage ÷ 60. Used-car depreciation is an assumption, not a guaranteed resale forecast, and should be adjusted for mileage, history, demand, and condition.
• The monthly planning range is a rough variance band: the larger of $45/mo or ±16% of variable ownership lines (insurance, maintenance, repairs, tires, and depreciation). It is not a statistical confidence interval; it is a reminder to quote and inspect before buying.
• Maintenance, repair reserve, tires, insurance, registration, taxes, and fees are planning reserves. Replace defaults with quotes, inspection findings, local DMV/tax rules, and real shop estimates before buying.
• Every numeric input is clamped to finite, non-negative values so invalid entries cannot create NaN, negative payments, or impossible totals.
• Primary public sources: NHTSA vPIC for vehicle identity/spec metadata and EPA/FuelEconomy.gov for fuel economy. Heuristic ownership-cost bands are labeled as assumptions until original owner-cost data exists.
• CarCostMath does not know your credit approval, exact insurance profile, local tax jurisdiction, tire size pricing, repair history, battery condition, rust, accidents, title status, or inspection findings. Those can flip a close comparison.