Yep, the 15 MPG's your computer says you are getting is wrong. There is good news however...since you went bigger your tires revolutions per mile are down and your computer thinks you are traveling at a slower MPH and traveling shorter distances than you really are. Since that is the case the MPG reading on you computer is lower than what you are actually getting. I went from 255/75r17 to 295/70r17 (approximately 32" to approximately 33.25" ) which represents about a 3.7% increase in tire diameter (to be more accurate I would need to look up manufactures data on exact size and Rev's per mile). Using a GPS I tracked my mileage and how much I fuel I was putting in over 2 month. My computer is currently reading 15.4 MPG's while my calculations put me at 16.1 MPG's, which is approximately a difference of 3.8%.
