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I am noticing worse mileage than I was before too. For me...I think it might be the extra rubber on my wheels. When I got new wheels I went 255 all around so that is more surface area. But I noticed my mixed average wen from 23-23.5 down to 21-22.1...same driving habits same driving conditions.
Yes more surface area is going to create more drag and lower fuel efficiency. thats why all those crap box cars meant for good mpg all have tiny thin tires. Check out solar cars too, those things basically have bicycle tires to reduce drag.