Trouble getting archives to load as archives

[Davin Pearson]

The following code works for Emacs Version 20.7 and 21.3
to load archive files as archives:

(setq auto-mode-alist
      (cons '("\\.\\(arc\\|zip\\|lzh\\|zoo\\)\\'" . archive-mode)
            auto-mode-alist))

I tried the following code but it only works on ZIP files:

(add-hook 'find-file-hooks 'my-archive-hook)
(defun my-archive-hook ()
  (when (or (string-match "\\.zip$" (buffer-name))
            (string-match "\\.tar$" (buffer-name))
            (string-match "\\.gz$"  (buffer-name)))
    (archive-mode 1)
    ))

So my question is this: how do I get archive-mode to display archives
correctly for TAR and GZ files?