Movies aren't rated PG *because* they're about cars. The PG rating is given by the Motion Picture Association (MPA) based on the content of the film itself. A movie about cars might be rated PG if it contains some thematic elements, mild action, or comedic situations that the MPA deems unsuitable for very young children, but not so intense as to warrant a higher rating like PG-13 or R.
For example, *Cars* (the Pixar movie) is rated PG because it might have some scenes that are mildly frightening or intense for very young children, even though it's generally a lighthearted and family-friendly film. The rating is a guideline, not a perfect measure of suitability for every child.
