The 1991 Chevrolet Camaro Base Model bolt pattern is 5-120.7 mm. This means there are 5 lugs and the diameter of the circle that the lugs make up measures 120.7 mm or 4.75 inches across. To get a close measurement of your 5 lug bolt pattern without a bolt pattern tool you should measure from the center of one lug to the outer edge of the lug furthest from it.
