Dude, don't give up hope. Piss tests can be easily circumvented (by using someone else's piss, or using one of many piss-cleaning techniques) and better yet once you're 18 nobody can force you into taking a drug test of any kind unless you are convicted of a drug crime, not even your parents. The key is to cut your financial ties to them (the car, etc) so they can't use them as tools to make your behavior conform to their beliefs. Once you are independent you will not be obligated to cooperate with them and you can say "You know what, I'm going to smoke pot whether you like it or not. I can manage my own life now, and if you sic the cops on me I hope you're real proud of yourself for making your own son suffer in a jail cell for doing something he enjoys that doesn't hurt anybody except potentially in the long run himself."