Required by law is 100mm clearance from the lowest point of the car (sideskirts, bumpers, exhaust or whatever). The only thing that doesnt get included is flexible mud flaps. If they are a solid mud guard such as late model falcons etc these get included.
Also the law states that this clearance is to be had with half a tank of fuel and ALL seating positions filled.
Hope that helps.
Oh and this Avatar was given to me by a Mod of SAUWA....i could change it, but said person will just change it again LOL....that aside, please take my above comments seriously LOL