diff --git a/lisp/tar-mode.el b/lisp/tar-mode.el
index bd10737b548..4adad6fe374 100644
--- a/lisp/tar-mode.el
+++ b/lisp/tar-mode.el
@@ -696,7 +696,7 @@ appear on disk when you save the tar-file's buffer."
 	      (setq coding (or coding-system-for-read
 			       (and set-auto-coding-function
 				    (funcall set-auto-coding-function
-					     name (point-max)))))
+					     name (- end start)))))
 	      (if (or (not coding)
 		      (eq (coding-system-type coding) 'undecided))
 		  (setq coding (detect-coding-region start end t)))