It’s not a big deal if the car only changes position, not shape, through the shot. You can keyframe the mask shape parameter by moving the mask across the screen.
Watch the footage back and forth, looking for places where the motion or shape of the object changes (those are “points of inflection”). Add intermediate keys only at these points. Don’t go frame-by-frame, and don’t pick an arbitrary interval (like 10 frames) to add keys. AE can interpolate the mask shape between points of inflection, so don’t go adding mask points anywhere else.