Why would paying an additional $100 every month, on top of the regular payment, towards one's car be beneficial?

Making principal-only payments accelerates the payment of your loan and decreases the interest you pay over the life of the loan. For example, you can save almost $900 in interest by paying an additional principal-only payment of $100 a month on a 60-month loan for $20,000 with a 7% interest rate. You'll also payoff your car loan one year and one month faster with the extra $100 payment.

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