i think the 18" vs 16" wheels have a lot to do with it. i bet most of the guys with the higher mileage are running 16's with stock tire size.

your 285 60 18's (i run the same size) are heavier than the stock 245 60 16's AND considerably taller. remember the profile number (60) is not a direct measurement, it's a ratio that depends on the width.

i think the speedo is off by 6% with my terra grapplers of the same size. and most of the extra weight is now located further from the center of the wheel, which requires much more enegy to rotate.