Auto Loan Refinance Calculator

How Auto Refinancing Calculations Work

Refinancing an auto loan replaces your current loan with a new one at a (hopefully) lower interest rate or different term. The key calculation is whether the interest savings exceed any fees and offset the cost of extending your loan term.

Monthly payment formula:

M = P × [r(1+r)^n] ÷ [(1+r)^n − 1]

Where P = remaining loan balance, r = new monthly rate, n = new term in months.

Worked example:

Current loan:

• Remaining balance: $18,000
• Current rate: 8.9% APR → monthly rate: 0.7417%
• Remaining term: 36 months
• Current payment: $572/month

New refinanced loan:

• Same balance: $18,000
• New rate: 5.9% APR → monthly rate: 0.4917%
• New term: 36 months

New payment = 18,000 × [0.004917 × (1.004917)^36] ÷ [(1.004917)^36 − 1] New payment ≈ $547/month

Monthly savings: $572 − $547 = $25/month

Total savings over 36 months: $25 × 36 = $900

Break-even calculation:

If there are refinancing fees (title transfer, lender fees): typically $150–$300.

Break-even months = Total fees ÷ Monthly savings = $200 ÷ $25 = 8 months

The refinance pays for itself after 8 months.

The term trap, with numbers

Take the same $18,000 at 8.9% with 36 months left, and refinance it at 5.9% over 72 months instead of 36. The payment falls from $572 to $297, which looks like a triumph. But you pay $297 for 72 months, so the total is $21,417 against $20,576 for simply keeping the old loan. A better rate, a payment cut of almost half, and $841 worse off. Meanwhile you are in debt for three extra years on a car that is depreciating the whole time.

This is why the calculator asks for your remaining term separately from the new one. Comparing both loans over the same term hides the only mistake most people actually make here.

When NOT to refinance:

• Your current loan has a prepayment penalty that exceeds savings
• Extending the term reduces monthly payments but increases total interest paid significantly
• Your car’s value is less than the loan balance (underwater), and lenders may decline
• You plan to pay off the loan very soon, so break-even will not be reached

Best auto refinance timing: within the first 2 years of the original loan, before most interest has already been paid.