So I have a question about my theory, If I got bigger wheels with tires, and the circumference is bigger, would that mess with the speedometer readings or anything of that sort?
If the wheel/tire is lighter and the new circumference puts you further down the road with every rotation, there's always a possibility of increased MPG. My tires have a circumference .99% greater than factory so if I want a true mpg after hand-calc I should multiply the answer by 1.099 but I don't even hand-calc any more.]
Most likely though your larger tire will also be wider and probably a softer compound, both of which translate to more friction and less mpg..
I thought I had read somewhere that you can input the size of your tire in your car and the car would calculate the new circumference to correct the speedometer. I can't seem to find it, but it might be worth a call to the dealership to see if that's possible or not.