Not a 6.7 ,but a 5.9 & I lost about 1 1/2-2 mpg when I went to 35's Toyo's.
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2007 2500 5.9 CTD MegaCab 4X4 Patriot Blue, Carli Starter System, Thuren TrackBar, Carli L/T AirBags, 35/12.50/17 Toyo M/T's on Black H2 Wheels, Denver Offroad Front Bumper, Buckstop Rear Bumper, Wet Okole Seat Covers, WeatherTech Floor Mats, WeatherGuard Tool Box