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Prepaying a loan: Shouldn't the interest be recalculated like a shorter loan?
For the mortgage, you're confusing cause and effect. Loans like mortgages generally have a very simple principle behind them: at any given time, the interest charged at that time is the product of the amount still owing and the interest rate. So for example on a mortgage of $100,000, at an interest rate of 5%, the interest charged for the first year would be $5,000. If you pay the interest plus another $20,000 after the first year, then in the second year the interest charge would be $4,000. This view is a bit of an over-simplification, but it gets the basic point across. [In practice you would actually make payments through the year so the actual balance that interest is charged on would vary. Different mortgages would also treat compounding slightly differently, e.g. the interest might be added to the mortgage balance daily or monthly.] So, it's natural that the interest charged on a mortgage reduces year-by-year as you pay off some of the mortgage. Mortgages are typically setup to have constant payments over the life of the mortgage (an "amortisation schedule"), calculated so that by the end of the planned mortgage term, you'll have paid off all of the principal. It's a straightforward effect of the way that interest works in general that these schedules incorporate higher interest payments early on in the mortgage, because that's the time when you owe more money. If you go for a 15-year mortgage, each payment will involve you paying off significantly more principal each time than with a 30-year mortgage for the same balance - because with a 15-year mortgage, you need to hit 0 after 15 years, not 30. So since you pay off the principal faster, you naturally pay less interest even when you just compare the first 15 years. In your case what you're talking about is paying off the mortgage using the 30-year payments for the first 15 years, and then suddenly paying off the remaining principal with a lump sum. But when you do that, overall you're still paying off principal later than if it had been a 15-year mortgage to begin with, so you should be charged more interest, because what you've done is not the same as having a 15-year mortgage. You still will save the rest of the interest on the remaining 15 years of the term, unless there are pre-payment penalties. For the car loan I'm not sure what is happening. Perhaps it's the same situation and you just misunderstood how it was explained. Or maybe it's setup with significant pre-payment penalties so you genuinely don't save anything by paying early.