I found on the net an article that explaines really well how to control car design proportions on your sketches and projects. The quedstion is (for beginners) how do I start to control my car design proportions proposal?
The classic rule is “the wheel” and its size. Use the wheel to create the basic frame work into which you can start sketching your side outline proposal.
The images here published are a good example of what I am trying to explane, however I suggest you to click on “Style4cars” and discover the full article with excellent lesson and images on this topic.
Images via Style4cars