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You did take into affect that the tire diameter affects your fuel mileage calculations.
Larger than normal tires will make a speedometer indicate LOWER than actual speed; thus it will not give an accurate reading of how far you have driven (odometer).
The speedo might show 70, but you are really doing 73; therefore, you have driven 73 miles in one hour, not 70 as shown on the odometer. (These were just an example from my Jeep where I went two sizes larger, I have no idea how far off your speed is, but is probably is off something due to larger tires.)

In addition, larger diameter heavier tires take more energy to spin and will lower your MPG.