Go here and you will see the size difference. Take that difference in % and multiply that by how many miles you drive before filling up. This will be around 4% by what I can tell. Add what you get to your total mileage and that is the actual miles you have drove.
Example: You drive 250 miles before empty and then fill up at 19 gallons.
250 miles X 4% = 10 miles...... So that gives you 260 miles.
260 miles / 19 gallons = 13.68
Instead of: 250 miles / 19 gallons = 13.15
Little better. You also have to take into consideration the rolling resistance of the new tires + the weight difference + the fact that you have now numerically lowered your gear ratio. Around town bad ====== Interstate Good.