Methodology

Every engine on this site reuses one of six shared calculation models instead of each inventing its own formula. This page documents what each model actually computes, where its numbers come from, and what it does not attempt to do. The full versioned record ismethodology/SHARED_PRIMITIVES.md in the project source.

Fixed-rate amortization

For a financed amount, APR, and term, the monthly payment is the standard fixed-rate formula (P·r / (1-(1+r)⁻ⁿ)). We then run a full month-by-month schedule so we can show total interest, payoff timing, and the effect of extra payments. This is a planning model, not a lender's payoff quote: it does not model daily simple-interest accrual, exact payment timing, prepayment penalties, or a specific lender's allocation rules. Source: CFPB, auto-loan amortization.

Equity and depreciation projection

Equity is market value minus loan payoff. To project equity forward, we apply your own annual depreciation figure (dollars or percent, never both) to the value side and the fixed-rate amortization schedule to the payoff side, then find the first month value meets or exceeds payoff. We never claim a forecasted resale value; depreciation is always a number you supply or edit.

Ownership total cost

Total cost of ownership over a horizon sums depreciation, financing interest actually paid in that horizon, fuel or energy, insurance, maintenance, repairs, registration, and any transaction or financing fees you enter, without double-counting purchase price and depreciation. A paid-off vehicle correctly shows zero financing interest but still carries real depreciation and operating cost.

Lease payment and buyout

Base lease payment splits into a depreciation charge ((cap cost − residual) / term) and a rent charge ((cap cost + residual) × money factor). For any lease exit, extension, mileage, disposition, or purchase-right question, this site uses only the amount or method stated in your signed contract, it never reconstructs a lessor's early-termination formula or implies a fee is current or legally required. Source:CFPB, leasing versus buying.

Fuel, charging, and marginal driving cost

Gas cost is miles divided by MPG, times your gas price. Electric cost accounts for a home/public charging blend and an optional charging-efficiency loss factor. No fuel, electricity, or charging rate is fetched automatically, you always enter or edit it. Source: U.S. DOE Alternative Fuels Data Center.

Break-even and sensitivity

Where a closed-form solution exists we use it; otherwise we use a bounded binary search over an explicitly stated plausible range. When no sign change occurs in that range, the engine says so plainly rather than forcing a number. Every Decision or Reverse engine that declares a break-even is tested by round-tripping the solved value back through its forward calculation.

What this site does not do

No engine defaults to a live APR, gas price, electricity rate, tax rate, incentive, or dealer-fee figure. Every current-market number is either entered by you or explicitly labeled as a planning estimate you can change. See Disclaimer.