The 1987 Porsche 911 Carrera Targa bolt pattern is 5-130 mm. This means there are 5 lugs and the diameter of the circle that the lugs make up measures 130 mm or 5.12 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.
