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Very solid foundation, but a few things bug me:
The first thing that stands out to me are the tires -- they dont seem to be strongly defined within the isometric perspective. Unless you intend for them to bulge out a little, I suggest giving those hubcaps some horizontal symmetry.
If you look up pictures of cars (from front view), you will see that the sides of the body (e.g. the doors) actually round out a little and tuck back in at the bottom -- tires on a car are actually set inside cavities carved into these curves, and are slightly deeper than the doors (more or less; depends on the car
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The hood should start to dip starting from the bottom of the windshield and moving towards the headlights; this could be a straight line or a gradual curve, it's your call.
Maybe give that front bumper some more definition? Maybe let it jut out from the front a little more, just to clarify with the viewer that it is in fact "there".
I realize that some of the things I mentioned might be a byproduct of the style restrictions that you are working in.