Bigger tires will make it look like you have worse mileage. Consider this - if the new tires are 1.2x the circumference of the stock tires, then when the odometer thinks you drove 1.0 miles, you will have really driven 1.2 miles. So, if you calculate you are getting 10mpg, you are really getting 12mpg. Without doing the math, I doubt that your tires are anywhere near 1.2x the circumference of the stock tires, but you get the idea.