While the GPS would be the most accurate I would also say that it is going to be marginal in the comparison because your other tires also wore down as time went on. If your odometer said 130 and the gps said 135 then that should give you a multiplier (135/130) of 1.0384615. Since you are going farther than your odometer says you will be getting better mileage than indicated, roughly 3.85%. Instead of the 15 miles per gallon you were getting the error in odometer reading would only account for about 0.5 miles per gallon. Not sure about the additional improvement.