The mileage dosen't necessarly go down. You are actually traveling further than your tip is reading. I did a trip this weekend with 215/45/r17 and the trip was off about 7.5-8%. Measured MPG 27.9 x 1.08 = 30.132 (running 75). And you are also going faster than your think.
A different contact patch wouldn't change the mpg. The over diameter will. If you get a combo thats bigger than the stock you would be going further, with smaller, you would be going shorter than what the odometer says. now weight on the other hand makes a difference.