I think I see what’s going on, maybe this will help
for ($i = 1, $numInventory = count($inventory); $i < $numInventory; $i++) {
// for each row of the input, create a new post
$post_information = array(
'post_title' => 'test '.$i,
'post_type' => 'vehicle',
'post_status' => 'publish'
);
$postID = wp_insert_post( $post_information );
// the following assumes that the order of
// the columns in the csv matches
// the order of the list of field keys
for ($m = 0, $numSub = count($inventory[$i]); $m < $numSub; $m++) {
// use the field key for $m column
update_field($field_keys[$m], $inventory[$i][$m], $postID);
} // end for $m
} // end for $i
