OP, I would just get AlfaOBD and correct the tire size. It is VERY easy and corrects the Odometer, speedometer, and MPG calculations. You will then also be able to change a number of additional things. For example, on my RAM, I changed the running lights to the amber turn signals (looks SOOO much better), enabled the Off-road pages, and turned on the tailgate ajar warning.
On another note, I did a 3"front/2" rear lift/level and have 35" Yokohama Geolandar A/T G015 295/65r20 tires. My MPG averages 15.5-16.5 with mostly in town driving to/from work.
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