Research is definitely important. Collect multiple pictures of the car in different angles and study its proportion. By doing so, it's even possible to go for a stylistic look (so the size of the wheels isn't necessarily an issue, although they still need adjustments), as long as you are able to balance recognisable features with whatever style you want.
That said, I have a few thought on the present design myself:
* The bumpers and side skirts are too perpendicular. The bumpers slope back towards the bottom in real-life.
* The front wheel well needs to be move a little to the front (2-4 pixels to the left). It's placed too far into the back.
* To reiterate questseeker's comment, the rear portion of the body (after the doors) is too long. Shorten it by (2-4 pixels).