That looks nice! The only critique I would have is to watch your perspectives. I can't draw cars for shit for that very reason. The areas that sort of stood out to me are the front end (the driver's side front fender seems a little "pinched") and the rims are a little out of perspective.
But all in all, this is a really nice sketch! Out of curiousity, what medium are you using? I've seen a lot of designers working in alcohol-based markers (such as Pantone or Prismacolor), which from my (limited) experience are a bitch!
Please keep posting these, I'm very interested in seeing your work.
edit: as for your question, I don't really design cars, but I design paintjobs fairly often for my S10 project. The actual truck is often the product of my light box, then I just use it as a frame to build bodymods and paintjobs on.