I see what you mean. Although in this instance i dont think it would cause to much problems by using their template since it is relitavely close. I do find Mr. Clipart's outlines are not precise. I always double check with the manufacturer's specs and compare them to their outlines and the overall lenght along with the wheelbase can be off by as much as 3". Their window dimensions are totally off to the real vehicles.
I dont know how acurate other collections are but i think these vector outlines are good for general sketches and presentations but accurate measurements of the actual vehicle to be lettered should be taken before final production begins.