Post URL based on Custom Post Types variables

Instead of creating a new tag, I would suggest just writing code to create the slug on a per-post basis using ACF’s save_post function.

//Update Cars slug field:
function wpse_post_slug_updater( $post_id ) {

    if( get_post_type() == 'cars') {
        $my_post = array();
        $my_post['ID'] = $post_id;
        $my_post['post_title'] = esc_html(get_the_title($post_id));

        $my_new_slug = get_field('brand') . ' ' . get_field('license_plate');
        if(!empty($my_new_slug)){
            $my_post['post_name'] = sanitize_title($my_new_slug);
            wp_update_post( $my_post );
        }
    }

}

// run after ACF saves the $_POST['fields'] data
add_action('acf/save_post', 'wpse_post_slug_updater', 20);