Correct me if im wrong put putting more air into the engine will not get you better fuel mileage. The more air that gets in the more fuel you use.
This is all theory here, but IMO anything other than a u-bend delete wont help gas mileage that much. When you are cruising you are not giving off a lot of exhaust gas so the restrictions arent as big of a deal. I dont think you would ever make the money back in gas savings by buying headers.
Lose weight in the car, accelerate very slowly, dont use cruise control on hills, use the lowest AC setting you can or open windows, get an alt rewire and voltage booster. You could do the "Big 3" If you really wanted, dont know how much it helps. Proper tire pressure will probably help the most, and make sure your wheels are aligned.
Also, not sure if you have a supercharger or not, but if you do you could get a 4" Pulley if you dont care how fast it is.
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