A portal gear box assembly effective to lift and change the gear ratio of a wheel of an off-road vehicle includes a housing, an input gear sub-assembly, an output gear sub-assembly, and an idler gear sub-assembly effective for linking the input gear to the output gear. The input gear sub-assembly includes an input gear, the output gear sub-assembly includes an output gear, and the idler gear sub-assembly includes at least one idler gear and at least one removable adaptor effective for holding the idler gear in either a first position or a second position different from the first position. The sub-assemblies may also include bearings to facilitate movement of the gears.