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If you modify your vehicle you might subconsciously change your driving so that you get better milage to justify your modification. It is not an independent test.
If you modify your vehicle and your non-hypermiling spouse who neither knows nor cares about the modification gets (or doesn't get) a milage improvement, that is an independent test you can be sure of.
So change your attitude and view your spouse's driving as a reliable test method and thus a positive thing.
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