1. Stand further away and crop the image
When photographing vehicles it's tempting to try to get the vehicle to fill the entire photo. By doing that you will get a slight fish eye effect and the proportions won't be perfect. In this example if you take a few steps back and to the right, take the picture from a little further away and crop it, that will give you a better looking image.
2. Think about the sun
Lighting has to be the most important factor in taking a good photo. To get the best light on a vehicle make sure to park where the sun will be behind where you will stand or to the side. Morning and evenings give the best light. And watch those shadows of yourself.
3. Turn in the front wheels
It's a simple tip. If you turn the front wheels so that they are at 90 degrees to where you will be shooting from you'll not only show off the wheels but get a better looking picture overall.
4. Go down on one knee
You'll get a much better picture from lower down. If you don't want to get right down on the floor then kneel down and take the shot from there. If you look at the final image below you'll see a much better balance of tarmac and sky and the background.
This one almost goes without saying. You can't afford to spend hours photographing every vehicle...but have a think about any locations there could be withing a mile radius that may show off the vehicle better. That will only take 10 minutes.