You seem to have general concepts down, you might consider keeping an eye on proportion more tho at this early stage in your artistic endeavors. Once you master proportion, everything else looks far better.
The last ones here are easily your best, proportions, though not perfect, are easily good enough to let us see the desings themselves. You have a definate style of your own showing through, be interesting to see how this goes with more mundane vehicles such as C segment hatches etc (remembering the dimensions needed to keep a reasonable interior size.
Im very impressed! I was looking at your first drawings thinking ehh.... but the ones in that last post are truly some thing great! Would you care to elaborate on how you do your presentation? I think alot of our newer users can benefit from it.