TRANSPARENT BY DESIGN
How DealProof calculates the deal
Every result is an estimate built from the numbers the user enters. DealProof does not pull credit, value a vehicle, quote a lender, or decide whether a fee is lawful.
Out-the-door price
The main calculator first subtracts the entered discount or rebate from the asking price. It then estimates the taxable base using the user's choices about taxable fees and whether the trade reduces taxable price.
asking price − entered discount
maximum of (adjusted price + selected taxable fees − selected trade credit) and zero × entered tax rate
adjusted price + estimated tax + dealer/doc fee + title/registration + add-ons
Because states and transactions treat rebates, trades, and fees differently, the user controls the two tax toggles and must match them to the actual buyer's order.
Trade equity and amount financed
trade allowance − trade-loan payoff
A positive result reduces the estimated amount financed. A negative result increases it.
out-the-door price − trade equity − cash down
The result cannot fall below zero. It excludes unentered lender fees, insurance, deferred payments, and products.
Fixed-rate loan payment
For an amount financed P, monthly rate r, and number of monthly payments n, DealProof uses the standard amortizing-loan formula:
P × [r(1+r)ⁿ] ÷ [(1+r)ⁿ−1]
The monthly rate is APR ÷ 12. At 0% APR, the amount financed is divided evenly by the term. Estimated total interest equals monthly payment × number of payments − amount financed.
Loan comparison
Both offers use the same entered principal. DealProof calculates each payment, total of payments, and total interest, then identifies the lower lifetime cost. It does not account for prepayment, variable rates, origination charges, or payment dates.
Negative-equity impact
The tool compares the estimated new loan with and without the trade equity. For negative equity, it also calculates the payment stream attributable to the rolled balance and subtracts that principal to estimate interest on the old debt.
Dealer-fee impact
Dealer-controlled charges equal the entered doc fee plus add-ons. When marked taxable, estimated tax on those charges is added. The tool amortizes that combined amount at the entered APR and term to estimate monthly and lifetime impact. Government charges remain separate.
Affordability calculation
The affordability tool reverses the loan formula to estimate the principal supported by a payment ceiling. It adds cash down and trade equity to create a total deal budget, subtracts fixed fees, and backs estimated tax out of the remainder. Negative trade equity reduces the budget.
Deal Clarity Score
The score is an educational screening aid, not a quality rating or approval. It begins at 100 and applies rule-based deductions for entered dealer fees, add-ons, negative equity, long terms, elevated APR, high interest relative to principal, and no entered discount. It does not know the vehicle's market value, the buyer's credit, local fee rules, or product quality.
Rounding and verification
Internal calculations use full decimal precision. Most displayed money is rounded to the nearest dollar for readability, so a lender's cents-based schedule can differ slightly. Always verify the official buyer's order and financing disclosures before signing.
Method last reviewed September 3, 2026. Consumer context is informed by the FTC's buying-and-owning-a-car resources.