What This Calculator Does
When buying a new vehicle, manufacturers and dealers often offer two mutually exclusive incentives: a cash back rebate you can apply to reduce your purchase price, or a low promotional interest rate through the dealer's financing arm. You cannot take both. This calculator determines which deal saves you more money over the life of the loan.
The answer depends on the size of the cash back offer, the promotional rate compared to your available bank or credit union rate, the vehicle price, and the loan term. This tool runs both scenarios and tells you exactly which option costs less.
Inputs Required
- Vehicle Price: Negotiated purchase price of the vehicle
- Down Payment: Cash paid upfront regardless of which option you choose
- Loan Term: Number of months for the loan
- Cash Back Rebate: The dollar amount the manufacturer or dealer offers as a rebate (Option A)
- Your Bank APR: The interest rate you qualify for through your own bank or credit union (used for Option A)
- Dealer Promotional APR: The low rate offered by the dealer in lieu of the cash back (Option B)
Outputs Provided
- Better Deal: Which option saves more money overall
- Monthly Payment Comparison: Side-by-side monthly payments for Option A and Option B
- Total Interest Comparison: Total interest paid under each option
- Total Cost Comparison: Full out-of-pocket cost for each option
- Total Savings: How much the better option saves over the loan term
How the Calculation Works
Option A Loan = Vehicle Price - Down Payment - Cash Back Rebate
Option A Payment = Amortization at your bank rate
Option B Loan = Vehicle Price - Down Payment
Option B Payment = Amortization at dealer promotional rate
Total Cost = Monthly Payment x Term + Down Payment
Option A takes the cash rebate and finances the reduced balance at your bank's market rate. Option B skips the rebate but benefits from a much lower dealer rate applied to the full loan amount. The result is not always intuitive. A large cash back amount often wins even against a very low dealer rate, particularly on shorter loan terms.
How to Use the Calculator
- Enter the vehicle's negotiated purchase price and your down payment
- Select the loan term in months
- Enter the cash back rebate amount from the dealer or manufacturer
- Input your pre-approved rate from your bank or credit union
- Enter the dealer's promotional financing rate
- Read the verdict and review both payment breakdowns
Example Calculation
A buyer is offered a $2,500 cash back rebate or 1.9% dealer financing on a $35,000 vehicle. Their bank pre-approval rate is 6.9% for 60 months:
- Option A: Loan of $29,500 ($35,000 - $3,000 down - $2,500 rebate) at 6.9% = approximately $583/month, total cost ~$37,980
- Option B: Loan of $32,000 ($35,000 - $3,000 down) at 1.9% = approximately $561/month, total cost ~$36,660
- In this example, Option B (low rate) saves approximately $1,320 over the loan term
Changing the loan term or rebate amount shifts the outcome. Try different scenarios to see where the crossover point is.
Real World Scenarios
Large Cash Back on a Short Loan
A buyer plans to pay off the car in 36 months with a $5,000 rebate available. On a short term, the interest savings from the promotional rate are limited, and the large cash back reduces the principal significantly. Cash back often wins in this scenario because less time means less interest advantage from the low rate.
Small Rebate on a Long Loan
A buyer finances for 72 months with a $1,000 rebate versus a 0.9% dealer rate. Over a long term, the ultra-low rate generates compounding interest savings that easily outweigh the small rebate. The low rate wins decisively here.
High Bank Rate Scenario
A buyer with a 12% bank rate can access a 2.9% dealer promotional rate. Even if the cash back is $3,000, the difference in interest rates over 60 months is so large that the dealer rate almost always wins. The promotional rate provides the most value when the gap between it and your bank rate is widest.
Why This Calculation Matters
Dealers often present these two options verbally without showing you the full cost comparison. Without running the numbers, it is nearly impossible to know which deal is actually better. The monthly payment difference may be small, but the total cost difference over the full loan term can be significant. Always run this comparison before signing.
Common Mistakes to Avoid
- Choosing based on monthly payment alone: A lower monthly payment does not always mean lower total cost. Run the full comparison before deciding
- Not getting a pre-approved rate first: Without a competing rate from your bank or credit union, you cannot accurately evaluate the dealer's promotional offer
- Assuming the low rate is always better: On shorter loan terms or with large rebates, cash back often saves more than a low promotional rate
- Ignoring the rebate on the purchase price: The cash back reduces your loan principal, which means less interest accrues even when financed at a higher rate