You must have been inches from the car for that perspective. Even a stopped down wide angle lens will give you a shallow DOF if you're focusing on something that's 12 inches away. So I think, given how sharp the front of the car is, that your depth of field was just too shallow.
Let's assume you focused on the front fender, it was 12 inches from your lens, and you were at 10mm and f/8. You DOF extends from 8 to 21 inches, i.e. a total depth of field of only 1 foot.
The point is that the single most important variable in DOF is the distance from the subject you're focused on. Following closely behind is your focal length. Way in the background is your aperture. If you're going to focus very very close to something, you're going to need to compensate with a tiny aperture to get a deep DOF. Fortunately at 10mm focal length it's not hard to do that, but f/8 isn't narrow enough.
Look up a DOF calculator, like http://www.dofmaster.com/dofjs.html
, to get a sense of how to get the most out of your DOF.
Granted superwides doesn't mean super sharp, but if you contol for DOF, camera shake, and sharpening, you should still get acceptable sharpness.
...... Well nearly always
