Bad oil leak and black oily residue on the back of car?

I have a 1996 plymouth neon. I know that it has a very BAD oil leak. I have to put oil in it before I go anywhere bc it all leaks out very quickly. I also have a black oily residue on the back of my car. I have never seen any smoke coming out of the tailpipe and I know the oil is leaking and not burning bc I see it on the ground. The car still runs very well as long as you keep oil in it, it never overheats, but does tap when the oil gets low. I dont think it is a cracked head or anything bc like I said it never overheats and still runs good. Also still gets great gas mileage. Does anyone have an idea as to what this may be or where the leak may be coming from?? I would really appreciate the help!! THANKS!!