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I wouldn't dismiss his claims simply because of your personal experiences. Many users on this website have seen >30% improvement just by changing their driving habits. Maybe he is just better at hypermiling than your friend?
Also, most of the modifications that you mentioned don't do anything to improve a car's mpg, and some of those modifications can actually hurt the mileage. For instance, even high-quality aftermarket rims can hurt the mileage because they are (typically) designed to vent heat away from the brakes, and that can have a very negative impact on aerodynamics.
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