Custom query works but returns “Undefined Offset: 0”

Try like this :

$args = array( 
    'post_type' => 'car',
    'posts_per_page' => -1,
    'tax_query' => array(
        array(
            'taxonomy' => 'your category/taxonomy name for post type car',
            'terms' => 'term name/category name',
            'field' => 'slug'
        )
    )
);
$query = new WP_Query($args);
$results = $query->get_posts();
print_r($results);

Yes every post has different slug, even if you have given same name.
For this you can create taxonomy term like which you have given post name, and assign this term to post you have in this category/term.

Ex.

Create taxonomy term named Post Slug and assign it to both post and try my query with your respective parameters.