The point the camera focuses on, and the depth of field are two different things.
Having said that, I'm sure I'm opening myself up to corective comments from those with greater technical knowledge...
In simple terms, the camera will calculate the point the lens is focused on. The area in focus forward and aft of that point is controlled by the aperture. This is the depth of field.
Neither the K10D nor the K100D have the ability to know what the depth of field is that you want.
There are a number of ways you can check your depth of field. One is the aformentioned DOF table. Another is to use the DOF preview that the camera has. Some people use the optical methode (ie through the view finder) others use the screen method. Give it a try.
A gret way to learn about DOF is with an old lens. Back in the day, lenses had a DOF table printed on the barel. So, you could set your aperture to f8, and look at the guide on the barel. In the case of my 135 3.5, at f8, focused at 15 feet, the DOF is about 14.5 to 16 feet.
So for your car. At that angle of the car, I am going to asume the length you want to be focused is about 182" for a camery. So 15'
Looking at the guide on the lens...
@ 20' f22, I get a DOF from 16' to 27' .....not enough
@ 20' f32, I get a DOF from 15' to 30' .....plenty for the car. As long as the lens is focused to 20'
But, f32 might need a tripod. It might also not be the shapest for the lens.
@ f11 i can get a DOF from 30' to 50' (estimated) with a focus point at about 35'.
This might be sharper for the lens.
Hope this helps.