A roof rack and its catching the air profile does make a difference.
As for the tire diameter, I just went for a road trip, and "messed" with the tire size a bit at a time until it matched the GPS in speed and distance. The programmed value is written down somewhere, and isn't a match to the actual tire size. It's black magic to get perfect I guess.
2006, 4.7L, QTII, Toyo Wild Country M/T 265/70R17 tires, Ultrasport 175 wheels, Rocky Road 2.25'' lift kit, HID off-road lighting, Surco 50x60 roof rack, Foxwing awning, Superchips performance programming.
I'm getting 14-17 MPG highway--corresponding to 80 mph and 60 mph respectively.
In town, it's more like 10-12. My Fuelly average (including off-roading) has dropped to about 13.5 with all the mods.
'06 XK Limited, 5.7 Hemi, QDII, OME struts & HD springs, JBA UCAs, 285/70R17 BFG KM2, 4XGuard Sliders, 4XGuard Diff Guard, Fox 2.0 reservoir shocks, custom bumpers and Warn xi9000
Wish list: SFA, in-dash Windows 8 PC
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