Auto Loan Refinance Calculator

When auto refinance applies

Two situations commonly prompt a refinance comparison: market rates have fallen since the original loan was signed, or the borrower's credit profile has improved enough to qualify for a better rate. How much either is worth depends on the size of the gap and how many months are left to run, which is what this calculator prices.

What the savings figure includes

This calculator compares the remaining life of the current loan at its current rate against the same balance at the new rate over the same remaining years. The headline is the sum of every monthly difference between the two payments: a saving when the quoted rate is the lower one, an added cost when it is higher. Holding the remaining years identical in both scenarios is what produces a clean comparison; what that leaves out is set out below.

How the math behaves

The total is the gap between the two monthly payments multiplied by the number of months left, and each part of it behaves differently. Balance scales both payments equally, so it scales their gap and the total exactly, leaving the percentage rows unchanged. Term works almost entirely through the month count: the monthly gap itself drifts only slowly across terms, and not in one direction: it falls about 1.5% between a one-year and a two-year term, then rises about 15% from that low point out to ten years, a total drift of around an eighth across the whole span. Those proportions are the same at any balance. Rate level matters separately from the size of the drop, because payment is non-linear in rate: the same three-point gap returns about 12.5% more at 20% to 17% than at 9% to 6%. A small drop on a balance close to payoff returns little, because few months of difference remain to capture.

What moves the number most

At the sample figures on this page (a three-point drop with four years left), the sizes separate cleanly. Balance is exactly proportional: 10% less balance gives 10% less saving. Term sits close to proportional and drifts above it as the term lengthens: doubling from one year to two multiplies the saving by about 1.97, from two to four by about 2.05, and from three to six by about 2.11. The two rate levers are near-mirrors, with the current rate marginally the larger: adding a point to the current rate lifts the saving by about 34%, while taking a point off the quoted rate lifts it by about 33%. Those proportions hold at any balance, so they read the same in every currency.

The formula behind this

The calculator computes the standard amortised monthly payment at both rates over the same remaining months, then multiplies the per-month difference by the number of months remaining, using unrounded payments throughout. Results are estimates for illustration purposes only. The full formula is shown in the formula box below so the arithmetic can be checked against a spreadsheet.

Where the simple comparison breaks down

The calculation assumes the same remaining term at the new rate. If the new loan extends the term, the monthly payment drops further but total interest paid usually rises, so the savings shown here will overstate the real benefit. Refinance fees, prepayment penalties on the old loan, and any temporary credit-score impact from the application are also outside the scope of this estimate.

What this doesn't capture

The model assumes a steady plan from refinance close to payoff. Missed payments, fee changes, rate resets on promotional offers, and refinances that close at a different effective rate than the quote all move the outcome away from it. The figure is therefore a clean-run estimate: it describes what the rate difference alone is worth, before anything else that happens to the loan.