Originally Posted by
brendon
I could be wrong, but I don't think running with the tire correction changes real world MPG, just the way you (or your EVIC) figure it out.
If you are running uncorrected, the car travels farther than what the odo is reporting, for example it may report a trip of 200 miles. Do the correction and now the exact same trip will show on the odo as 250 miles. Say you used 20 gallons on each trip; according to the math the first trip you got 10mpg, but on the second trip you got 12.5mpg. Real world this trip distance as measured by a survey crew could be 230 miles giving a real MPG of 11.5 for both trips. The trip distance and fuel usage did not change, just the factors used to figure out the mileage.
Now, if the computer uses different cal factors for tire size to alter the way the Jeep puts the power to the road, then all bets are off...