In addition to Nathanael's good suggestions I would try the following:
1) Most of your pictures seem clumped together. Instead, stand at a distance where you can get the entire car in one shot, and start taking pictures every 5-10 degrees, while keeping a constant distance from the car.
2) Try to take the pictures facing slightly down. This will catpure some of the ground, and this is usually textured (and so has a lot of features that are easily synthed).
3) Take all the pictures in one location. Having different backgrounds will confuse Photosynth.
4) Keep the roof down. There's more features on the inside of the car.
5) For object like cars (singe point of interest), a reasonable rule of thumb is to keep moving while taking pictures. Taking more than one pictures in the same place will stitch, but not help the general reconstruction that much. Moving the camera gives parallax.