I think your problem is that all your drawing look like we are looking down on the vehicle from a third floor balcony. You tend to elongate your vehicles and overly narrow the backends.
Why don't you try a more common perspective. Don't draw your cars from an upper view. Draw a 3/4 shot as if you were standing next to the front fender. You should be able to see some of the hood but very little of the roof panel. From this angle, you shouldn't have to worry about vanishing points or perspective at all.
Also, TAKE YOUR TIME! I don't post chops very often, but when I do I normally have spent MANY HOURS working on them, normally over several days. Keep refining your design until it is right.