Free Auto Loan Payoff calculator with clear step-by-step results.
Solves the amortisation formula for the number of months rather than the payment, so you can see how much sooner a car loan clears when you add to the scheduled instalment - and how much interest that acceleration actually saves.
Months remaining
n = -ln(1 - r x B / P) / ln(1 + r), where r is the monthly rate, B the balance and P the payment
Interest saved
Saved = (base payment x base months - B) - (accelerated payment x accelerated months - B)
The balance would never fall, so the calculation caps the term at 600 months to flag it. You need a payment above balance x monthly rate for the loan to amortise at all.
Not automatically. Some hold the surplus as a prepaid instalment, which does not reduce interest. Tell the lender in writing to apply extra amounts to principal.