The 1984 GMC C20 Pick-up 2wd bolt pattern is 6-139.7 mm. This means there are 6 lugs and the diameter of the circle that the lugs make up measures 139.7 mm or 5.5 inches across. To get a close measurement of your 6 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.
