Allow me, if you will, to add my perspective to this.
I had a "similar" dealings with my mum, albeit not quite the same, but its similar in principle.
My mum went through a phase of being overprotective, in that she was uncomfortable about letting me go out.
I confronted her and asked her for a legitimate reason, of course the usual came about, "its not safe" and "you're my only son" etc...
By keeping calm and rational, I told her that these things were ultimately my decision, and it was my money etc, all the things people have already told you in this thread.
Now I can tell her where I'm going and what I'm doing, there be no cloak and dagger stuff here.
Also the fact that I saved her from certain death at the hands of the mighty shredder may have helped my case. *
My sister on the other hand, has not spoken to her in the same way, and as such is playing up moreso because of it.
Lieing about where she's going, doing more crazy things and generally messing around more.
You're 18, and you need to have this conversation sooner or later, and make her realise whats going on.
It'll be hard for her I'm sure, since you're an only child but if you reason with her, she'll see sense and acknowledge the situation. However the whole, lack of ride thing is a problem.
Ah well, you'll sort it out.
* statement could well be falsified