diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 9d38421..8549677 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -245,8 +245,8 @@
        the high bit set. So we calculate two checksums, unsigned and
        signed.
     """
-    unsigned_chksum = 256 + sum(struct.unpack("148B", buf[:148]) + struct.unpack("356B", buf[156:512]))
-    signed_chksum = 256 + sum(struct.unpack("148b", buf[:148]) + struct.unpack("356b", buf[156:512]))
+    unsigned_chksum = 256 + sum(struct.unpack_from("148B8x356B", buf))
+    signed_chksum = 256 + sum(struct.unpack_from("148b8x356b", buf))
     return unsigned_chksum, signed_chksum
 
 def copyfileobj(src, dst, length=None):