Hi koganei_hikaru
Like some mentioned before, You have one tone for the entire car so it makes the car look flat and not 3D.
Thom Taylor's book How to Draw Cars Like a Pro talks about shading w/ the 123 rule. In most cases you have 3 over all tones on a car due to the single source of light (the sun, if outside). Go to Barns&Noble and check out his book, read the chapter 7 titled, "Light Source" pages 52-53. You might want to buy the book (HC) online if B&N doesn't have it in the store.
http://search.barnesandnoble.com/boo...NL&cds2Pid=946
Get the hard cover. Only B&N carries it (hard cover).
Al