put some 17s and 40/45 tires on, then push your car in neutral.
put your stock 15s and 60/65 tires on, then push your car. you will see that it's a hell of a lot easier to roll car with 17s. even with the stock tires fully inflated, it's still a big mushy balloon and you will get crappy gas mileage..that's my theory...and the overall wheel diamter will affect your MPG too.