Methodology
Methodology v1.0 organizes user-entered estimates into a cash-flow comparison. It does not verify a diagnosis, predict future repairs, or calculate complete economic ownership cost.
What the headline comparison means
The headline compares estimated cash paid during the selected 12-, 24-, or 36-month period. It uses the values the user enters. It is decision support—not mechanical, safety, legal, insurance, purchasing, or personalized financial advice—and it should not be read as a complete cost of ownership.
Repair-and-keep cash flow
The repair path adds the entered repair quote, expected future maintenance and repairs, and current vehicle loan payments due during the comparison period. Current loan payments equal the entered monthly payment multiplied by the lesser of the comparison months or the number of payments remaining.
If expected future maintenance and repairs is $0, no automatic reserve is substituted. The result warns that this may make keeping the current vehicle appear less expensive. The calculator does not project the current vehicle’s ending value or loan payoff because the inputs are not enough to do that accurately.
Current vehicle equity and replacement financing
Current vehicle equity equals estimated current vehicle value minus current loan payoff. Positive equity is treated as value available toward the replacement. Negative equity is added to the estimated replacement financing, so an unpaid current-car payoff does not disappear from the replacement scenario.
Replacement cash flow includes the entered down payment, taxes and fees, modeled replacement-loan payments during the comparison period, and entered monthly insurance, fuel, and maintenance differences. Taxes and fees are treated as upfront cash and are not also added to the financed amount.
If the current vehicle value is unknown, the calculator does not apply positive sale or trade-in value. It marks the result as limited information and explains that replacement financing and repair-to-value context are provisional. A broad value range uses its disclosed midpoint as a planning assumption.
Loan payments, balances, and equity
Loan payments include both principal and interest. Principal payments can build vehicle equity and are not identical to an ownership expense. For replacement loans, the calculator uses the entered price, down payment, equity, APR, and term to estimate payments and the remaining balance at the end of the comparison period.
Upfront cash, monthly cash-flow equivalent, remaining loan balance, vehicle value, and vehicle equity are shown as separate categories. They should not be added together without a complete economic-cost model.
Ending value and depreciation
Replacement depreciation is only estimated when the user enters an ending vehicle value. It equals purchase price minus ending value and is clamped at $0, so the calculator never reports negative depreciation. Ending equity equals the entered ending value minus the estimated replacement-loan balance at that time.
Depreciation and ending equity are displayed separately and do not change the headline cash-flow total. When no ending value is entered, depreciation is excluded and the result warns that replacement may appear less expensive. The replacement total is not presented as complete ownership cost.
Close calls and result stability
The calculator compares repair cash flow with the lowest replacement cash flow. A close call is reported when the absolute difference is no more than the larger total multiplied by 10%, or $750, whichever is greater. This threshold is a result-stability setting, not an industry standard.
The product labels a result More stable, Sensitive, or Limited information. This is not a probability or a prediction that a repair will succeed. It describes whether realistic input changes and missing evidence could change the leading financial option. Repair-evidence answers do not change the entered repair amount. Safety uncertainty overrides the financial label so the result asks for professional review first.
What can change the result
Unknown future repairs, resale values, financing terms, taxes, insurance, fuel costs, maintenance, repair warranties, downtime, and transportation needs may change the outcome. The calculator does not inspect the vehicle, verify a repair price, look up market values, or obtain lender and insurer quotes.
Before acting
Verify the diagnosis, itemized estimate, repair warranty, other near-term work, replacement price, financing, current payoff, vehicle values, insurance, taxes, and fees that matter to your situation. Financial comparisons should not override safety.
Version history
v1.0 · July 19, 2026: Initial published version. Defines the three-option cash-flow comparison, close-call threshold, safety override, current-value unknown state, and result-stability explanation.