The 2008 Honda Civic Sedan SI bolt pattern is 5-114.3 mm. This means there are 5 lugs and the diameter of the circle that the lugs make up measures 114.3 mm or 4.5 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.
