Maybe 1" of difference in wheel diameter, but the tire OD difference is .1" (the 265/65/18 being taller). This translates to a .2 MPH difference at 60 mph.

 

By my math, your going an extra 3.4ish inches per rotation. Your old tires went 661 rotations per mile, your new ones go 638 rotations in a mile. A difference of 78.2 inches per mile. Now that over, say 300 miles a tank, is about 23,460 inches or 1955 feet. So based of this, your mileage should change very little, just on the math. But when you add in the extra weight, and taller over all tire size you lose mechanical advantage which hurts most durring acceleration, plus your gear ratio makes it worse. On top of that you now sit a little higher which means more drag.

 

And thats basically what I was getting to. All factors on a singular basis, is relatively small, but things add up to to worse issues.