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mortgage vs car loan vs invest extra cash?
It depends on your tax rate. Multiply your marginal rate (including state, if applicable) by your 3.1% to figure out how much you are saving through the deduction, then subtract that from the 3.1% to get the effective rate on the mortgage. For example, if you are in the 28% bracket with no state tax impact from the mortgage, your effective rate on the mortgage is 2.232%. This also assumes you'd still itemize deductions without the mortgage, otherwise, the effective deduction is less. Others have pointed out more behavioral reasons for wanting to pay off the car first, but from a purely financial impact, this is the way to analyze it. This is also your risk-free rate to compare additional investing to (after taking into account taxes on investments).