THE NUMORIX GUIDE
How to use the Cash Back or Low Interest Calculator
Last reviewed September 14, 2026
What this calculator does
The comparison amortizes two vehicle offers over the same month term.
Formula and method
The comparison amortizes two vehicle offers over the same month term. The low-interest offer uses lowInterestPrice minus down payment; the cash-back offer uses carPrice minus cashBackAmount minus down payment. Each offer receives its own annual rate, and total cost adds down payment to all scheduled monthly payments. The lower total is recommended.
Variables and inputs
The UI exposes car price, down payment, term in months, low-interest rate, cash-back amount, and cash-back rate. The engine also has lowInterestPrice, which remains at its default unless another caller changes it; the current form does not show that field. Money values are dollars and rates are percentages.
Worked example
With $35,000 price, $5,000 down, 60 months, 2.9% low-interest rate, $3,000 cash back, and 5.9% cash-back rate, the low-interest principal is 35,000 - 5,000 = $30,000. The cash-back principal is (35,000 - 3,000) - 5,000 = $27,000. Compare the two amortized payments and add $5,000 to each payment total; the lower total is the modeled better offer.
How to interpret the result
The recommendation is a financing-cost comparison, not a complete vehicle purchase comparison. It assumes the offers have the same term, down payment, fees, taxes, and vehicle price except for the entered incentive and rate differences.
Common mistakes to avoid
Hold out-the-door price constant when comparing real offers. Do not subtract cash back from one offer and also count it as cash received elsewhere. Check the hidden low-interest price assumption if the low-rate offer has a different selling price.
Assumptions and limitations
The current UI does not expose lowInterestPrice, taxes, fees, trade-in value, or other vehicle costs. The engine also rounds results to whole dollars and treats cash back as an immediate price reduction. Verify the actual contracts and rebate eligibility.