The 2005 Chevrolet Silverado 3500 4wd Crew Cab Dually bolt pattern is 8-165.1 mm. This means there are 8 lugs and the diameter of the circle that the lugs make up measures 165.1 mm or 6.5 inches across. To get a close measurement of your 8 lug bolt pattern without a bolt pattern tool you should measure from the center of one lug to the center of the lug across from it.
