Well, your perspective isn't that bad. Well, to me at least, who is really having problem making the drawings to come out right. I don't think yours suffers too much of perspectives. You should have seen mine, they are much worse.
Sometimes what i do to get some general proportions is throw a wallpaper car image on the bottom layer of photoshop, outline the outside of the tyres put a dot on the roof windscreen bonnet and front bar... this helps it stay in proportion, and you can still make the car look however you like!