depends on the year, on the OBS trucks fronts are 6.5, rears are 4X6, on NBS the fronts are 6.5 and the rears are 5.25, though i have fit 6.5 in there before with no modification. NNBS i would have to double check but i recall there being a 6.5 option for both doors and a 6X9 option for both...