Put all of your driving stuff on, helmet and hans. Get in the car and have someone measure the distance between the back of your helmet and front of the down tubes of the roll cage. ISP told me their standard rear helmet pad is 7x7, 3 inches deep and they use zip ties to mount the pad to the down tubes of the cage. Give ISP a call they're great to deal with and can answer any questions you have. My pad new from ISP cost $100.