GPT-5.6 Terra vs Grok 4.5for everyday maths

Early overpayments cut the mortgage balance when it is highest, so every future monthly interest charge is calculated on a smaller amount. That saving then compounds over many years: less interest builds up, and more of each normal payment goes toward the loan itself. In the final five years, there is less balance left and far less time for reduced interest to accumulate, so the same £100 overpayment has a smaller overall effect.

Overpaying early slashes the large starting balance sooner. Interest builds on whatever you still owe, so cutting the debt early prevents decades of interest from accumulating on those extra payments. Near the end the balance is already tiny and little time remains, so the same £100 saves almost nothing.

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