I found another post with the solution.
add_action( 'save_post', 'set_featured_image_from_gallery' );
function set_featured_image_from_gallery() {
$has_thumbnail = get_the_post_thumbnail($post->ID);
if ( !$has_thumbnail ) {
$images = get_field('car_gallery', false, false);
$image_id = $images[0];
if ( $image_id ) {
set_post_thumbnail( $post->ID, $image_id );
}
}
}