-18 is common offset for 10" wide wheels. I'm guessing thats what it is. They'll be out 3" further than stock and will require some modification to make fit. IE level and trimming/cutting. This truck in the link has 10" wide with -19 so essentially the same fitment.
http://www.customwheels4u.com/gallery/23823/ford-f-150-raptor-fuel-2-piece-wheels-nutz-d251/
http://www.customwheels4u.com/gallery/23823/ford-f-150-raptor-fuel-2-piece-wheels-nutz-d251/