I think what Ruken is thinking is that making the extra payment will reduce the interest paid on that loan. Since the consumer loan (the truck) isn’t deductible anyway, nothing changes on your tax return by reducing the interest you pay on that loan. The interest on the mortgage is deductible, so reducing your mortgage interest will be slightly offset by the lost benefit of the tax deduction.
And this highlights what the OP should really focus on: total after-tax cost of debt. The truck loan is 5.5% before and after tax. The mortgage is 4% before tax, but only 3% after tax (assuming a 25% tax bracket). We could say, then, that the after-tax savings of paying off the auto loan first is 6% of the amount paid per year, and a marginal after-tax savings of 2.5% compared to paying on the mortgage.
It’s worth checking to make sure that both rates are fixed. If one has a variable rate or a balloon payment, that might be a more important factor than the present interest values.