diff --git a/src/calibre/gui2/preferences/tweaks.py b/src/calibre/gui2/preferences/tweaks.py
index dbc917131d..de18f88e34 100644
--- a/src/calibre/gui2/preferences/tweaks.py
+++ b/src/calibre/gui2/preferences/tweaks.py
@@ -1,43 +1,35 @@
 #!/usr/bin/env python2
 # vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai
+# License: GPLv3 Copyright: 2010, Kovid Goyal <kovid at kovidgoyal.net>
 
-from __future__ import print_function
-__license__   = 'GPL v3'
-__copyright__ = '2010, Kovid Goyal <kovid@kovidgoyal.net>'
-__docformat__ = 'restructuredtext en'
 
-from functools import partial
 import textwrap
 from collections import OrderedDict
-
-from calibre.gui2.preferences import ConfigWidgetBase, test_widget, AbortCommit
-from calibre.gui2.search_box import SearchBox2
-from calibre.gui2 import error_dialog, info_dialog
-from calibre.utils.config_base import read_custom_tweaks, write_custom_tweaks, exec_tweaks, default_tweaks_raw
-from calibre.gui2.widgets import PythonHighlighter
-from calibre import isbytestring
-from calibre.utils.icu import lower
-from calibre.utils.search_query_parser import (ParseException,
-        SearchQueryParser)
-from polyglot.builtins import iteritems, unicode_type, range, map
+from functools import partial
 
 from PyQt5.Qt import (
-    QAbstractListModel, Qt, QStyledItemDelegate, QStyle, QStyleOptionViewItem,
-    QFont, QDialogButtonBox, QDialog, QApplication, QVBoxLayout,
-    QPlainTextEdit, QLabel, QModelIndex, QMenu, QIcon, QListView, QGridLayout,
-    QSizePolicy, QGroupBox, QWidget, QPushButton, QSplitter, pyqtSignal)
+    QAbstractListModel, QApplication, QDialog, QDialogButtonBox, QFont, QGridLayout,
+    QGroupBox, QIcon, QLabel, QListView, QMenu, QModelIndex, QPlainTextEdit,
+    QPushButton, QSizePolicy, QSplitter, QStyle, QStyledItemDelegate,
+    QStyleOptionViewItem, Qt, QVBoxLayout, QWidget, pyqtSignal
+)
+
+from calibre import isbytestring
+from calibre.gui2 import error_dialog, info_dialog
+from calibre.gui2.preferences import AbortCommit, ConfigWidgetBase, test_widget
+from calibre.gui2.search_box import SearchBox2
+from calibre.gui2.widgets import PythonHighlighter
+from calibre.utils.config_base import (
+    default_tweaks_raw, exec_tweaks, normalize_tweak, read_custom_tweaks,
+    write_custom_tweaks
+)
+from calibre.utils.icu import lower
+from calibre.utils.search_query_parser import ParseException, SearchQueryParser
+from polyglot.builtins import iteritems, range, unicode_type
 
 ROOT = QModelIndex()
 
 
-def normalize(val):
-    if isinstance(val, (list, tuple)):
-        return tuple(map(normalize, val))
-    if isinstance(val, dict):
-        return {k: normalize(v) for k, v in iteritems(val)}
-    return val
-
-
 def format_doc(doc):
     current_indent = default_indent = None
     lines = ['']
@@ -120,7 +112,7 @@ class Tweak(object):  # {{{
     def is_customized(self):
         for x, val in iteritems(self.default_values):
             cval = self.custom_values.get(x, val)
-            if normalize(cval) != normalize(val):
+            if normalize_tweak(cval) != normalize_tweak(val):
                 return True
         return False
 
diff --git a/src/calibre/utils/config_base.py b/src/calibre/utils/config_base.py
index 6b177a8e02..0434abfc0e 100644
--- a/src/calibre/utils/config_base.py
+++ b/src/calibre/utils/config_base.py
@@ -539,9 +539,24 @@ def make_unicode(obj):
     return obj
 
 
+def normalize_tweak(val):
+    if isinstance(val, (list, tuple)):
+        return tuple(map(normalize_tweak, val))
+    if isinstance(val, dict):
+        return {k: normalize_tweak(v) for k, v in iteritems(val)}
+    return val
+
+
 def write_custom_tweaks(tweaks_dict):
     make_config_dir()
-    raw = json_dumps(make_unicode(tweaks_dict))
+    tweaks_dict = make_unicode(tweaks_dict)
+    changed_tweaks = {}
+    default_tweaks = exec_tweaks(default_tweaks_raw())
+    for key, cval in iteritems(tweaks_dict):
+        if key in default_tweaks and normalize_tweak(cval) == normalize_tweak(default_tweaks[key]):
+            continue
+        changed_tweaks[key] = cval
+    raw = json_dumps(changed_tweaks)
     with open(tweaks_file(), 'wb') as f:
         f.write(raw)