I believe it does both. Your overhead will be further off but you should still see the Improvement in the actual MPGs.
But we all should end up in the same approximate place right?all the checking i'm doing , it seems like the guys who had better fuel mileage from the start see less of a gain vs. the people who have crap at the beginning and see bigger gains. Is this possible? Could some trucks come with more pressure or a better tune?