How can I get Dad to let me drive one of his extra cars?

I'm supposed to be getting my license soon, and I've been looking for a car. But Dad has a whole bunch of cars just sitting around rusting away. There's one that runs great, and he stopped driving it because he got ANOTHER truck. He only drives this car when he has to pick up the rest of the family to go somewhere. (I live with my mom and my sister and he lives across town.) Since he doesn't use it, I thought I'd ask if I could drive it once I get my license. But he says no. Don't even tell me to get my own car, because I've been trying that. I'm self-employed, and my meager income doesn't allow for it. I should be having a huge sale tomorrow, so that should help, but what can I say to Dad to have him let me drive his car? He says that if he lets me drive his car, I'll do drugs... I don't understand. I don't have any wish to do drugs. Even though I have a few friends who smoke ciggarettes, I don't even want to do that. I'm very responsible, and I've never done anything majorly stupid. He just doesn't trust me for some reason.