Get or make you some circle stencils.
It helped me tremendously. Draw the line where the wheels touch the ground. (make a box around where you want the car to sit) Then half circle in the wheel openings, then draw in your wheels under them, and depending on how you want the car to sit, adjust the "turn" of the wheel. (Look at some car pics in a magazine) Try drawing your wheels in accordance with the stance of the car kinda. How the car sits makes the drawing. I know it kinda sucks drawing wheels sometimes, but they are what helps the rest of the car come together proportion-wise. Kinda like your guidelines, along with the box method.
Keep trying, Practice, Practice, Practice. It will pay off, it did for me.