I have a math question and I wasn’t sure the best place to ask it.

Let’s say I get a mortgage for $229000 at 2.5% interest for 30 years. Based on an amortization schedule, my payment will be ~$905 with $428 to principal and $477 to interest on my first payment. I’m trying to figure out how much extra payments save me, and I’m looking at it two ways getting two different answers. Where’s the disconnect?

Let’s say I make a principal only payment on day 1 of $428. That basically moved my amortization schedule to the left one month. So in 30 years, I won’t have to make my last payment. So $428 today saves me $905 in 30 years.

Another way to look at this is that each month I’m paying interest on that extra $428. So let’s just look at the interest rate as if that $428 is a separate loan. I pay 2.5% interest each year for 30 years. Let’s say I pay interest only payments each month so there’s no compounding or anything like that. That’s 428*30*2.5/100 = 321 plus my original payment for a total of 428+321 = 749. So paying $428 today will save me $749 over 30 years.

That’s not a small discrepancy. What am I missing?