Prepaying a loan: Shouldn't the interest be recalculated like a shorter loan?
One way to think of the typical fixed rate mortgage, is that you can calculate the balance at the end of the month. Add a month's interest (rate times balance, then divide by 12) then subtract your payment. The principal is now a bit less, and there's a snowball effect that continues to drop the principal more each month.  Even though some might object to my use of the word "compounding," a prepayment has that effect. e.g. you have a 5% mortgage, and pay $100 extra principal. If you did nothing else, 5% compounded over 28 years is about 4X. So, if you did this early on, it would reduce the last payment by about $400. Obviously, there are calculators and spreadsheets that can give the exact numbers.  I don't know the rules for car loans, but one would actually expect them to work similarly, and no, you are not crazy to expect that. Just the opposite.
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Prepaying a loan: Shouldn't the interest be recalculated like a shorter loan?

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One way to think of the typical fixed rate mortgage, is that you can calculate the balance at the end of the month. Add a month's interest (rate times balance, then divide by 12) then subtract your payment. The principal is now a bit less, and there's a snowball effect that continues to drop the principal more each month.  Even though some might object to my use of the word "compounding," a prepayment has that effect. e.g. you have a 5% mortgage, and pay $100 extra principal. If you did nothing else, 5% compounded over 28 years is about 4X. So, if you did this early on, it would reduce the last payment by about $400. Obviously, there are calculators and spreadsheets that can give the exact numbers.  I don't know the rules for car loans, but one would actually expect them to work similarly, and no, you are not crazy to expect that. Just the opposite.