I had to go searching to find out what "real" focal lengths your camera has (not the 35mm equivalent, but the actual focal length of the lens) and found here that it goes from 5.7mm to 57mm (37mm to 370mm is the 35mm equivalent based on the size of your sensor).
Okay, so knowing what the real focal lengths are, let's take a look at an Online Depth of Field Calculator to see what your Depth of Field ranges are. Select your camera from the drop down list. Then select a focal length setting of 5.7mm and an aperture of f/2.8. On the right-hand side of the screen you can see that your hyperfocal distance is 7.56 feet. In other words, at f/2.8, if you focus on something 7.56 feet away, then it and everything beyond it (to infinity) will all be in focus.
Stopping down to f/8, the hyperfocal distance drops to 2.68 feet.
In other words, at the wide end of your lens (5.7mm), there is very little reason to ever shoot anything less than wide open because you almost always have everything in focus. (side note: however, most lenses are sharper at a stop or two down from wide open, but this has nothing to do with Depth of Field)
Now let's look at the long end of the lens. Zooming out to 57mm we get a bit of a different story. Once again, set the aperture to f/2.8 and look at the hyperfocal distance. This time you'd have to focus on an object 727 feet away in order to get the subject and everything behind it (to infinity) in focus, or 257 feet at f/8.
I doubt you were shooting cars that far away. So let's get some more realistic numbers and look at what the DOF looks like. Let's say you were using 57mm and your subject was 50 feet away. If you focus on your subject, then everything from 46.8 feet to 53.7 feet will be in focus (i.e. a DOF of 6.88 feet). So as long as your subject isn't deeper (front to back) than 6.88 feet long (and you focus in the middle of it), you will get your entire subject in focus at 50 feet away, using a 57mm focal length at f/2.8.
Changing to f/8 increases the depth of field. Now everything from 41.9 to 62 feet away will be in focus (20.1 feet).
Chances are, you have no need for f/8 at a subject distance of 50 feet when shooting cars. So instead, save the f/8 setting for landscape shots (when zoomed all the way).
Hope this helps.
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