diff --git a/assets/contracts/checklists/z27ag-captures.json b/assets/contracts/checklists/z27ag-captures.json
index 49126bd..ab84ae8 100644
--- a/assets/contracts/checklists/z27ag-captures.json
+++ b/assets/contracts/checklists/z27ag-captures.json
@@ -1,6 +1,6 @@
 {
   "id": "z27ag-captures",
-  "version": 3,
+  "version": 4,
   "title": "Z27AG decoding captures",
   "about": "docs/impl/plans/Z27AG-CAPTURES.md",
   "mark_format": "<step> <action>[ #<n>] start|end for a span, <step> <action>[ #<n>] for a point",
@@ -445,6 +445,30 @@
               "hold_s": 10
             }
           ]
+        },
+        {
+          "id": "31",
+          "title": "One wheel at a time, by hand",
+          "do": "Ignition on, engine off, neutral. A front: hold the other front still (open differential: in neutral the turned wheel spins the gearbox, not its partner) and spin this one briskly by hand, forward, about one turn a second; start as it turns, end as you stop. A rear, if you raise it: rears turn freely, no holding needed. Fronts are enough if the rears are not up.",
+          "finds": "which word of 235 is which wheel, one at a time (the drives already point to bytes 0-1 front right, 2-3 front left, 4-5 rear right, 6-7 rear left); also the slowest speed the ABS reports",
+          "actions": [
+            {
+              "label": "front left by hand",
+              "hold_s": 10
+            },
+            {
+              "label": "front right by hand",
+              "hold_s": 10
+            },
+            {
+              "label": "rear left by hand",
+              "hold_s": 10
+            },
+            {
+              "label": "rear right by hand",
+              "hold_s": 10
+            }
+          ]
         }
       ]
     },
diff --git a/lib/main.dart b/lib/main.dart
index b836d44..8fc5644 100644
--- a/lib/main.dart
+++ b/lib/main.dart
@@ -5,6 +5,7 @@ import 'package:flutter/material.dart';
 
 import 'link/boundary.dart';
 import 'link/net.dart';
+import 'link/reading.dart';
 import 'pages/link_test.dart';
 import 'shell.dart';
 import 'shell_scope.dart';
@@ -24,6 +25,14 @@ Future<void> main() async {
   final usb = UsbLink();
   await usb.bindFor(Uri.parse(defaultUrl));
   final boundary = Boundary(defaultUrl)..start();
+  // The sun, for auto dark/bright: the car's position while GNSS has one.
+  boundary.addListener(() {
+    final lat = Reading.of(boundary, 'gnss_lat');
+    final lon = Reading.of(boundary, 'gnss_lon');
+    if (!lat.dim && !lon.dim && lat.number != null && lon.number != null) {
+      themes.setPosition(lat.number!, lon.number!);
+    }
+  });
   final linkTest = LinkTest(usb);
   // Development hook: `am start ... --es linktest [dl:|bi:]<label>[@<host>]` runs the test
   // (`dl:` the download series, `bi:` burst latency against idle gaps), on the gateway's host unless one is given.
diff --git a/lib/pages/settings.dart b/lib/pages/settings.dart
index 927862d..45aa5b2 100644
--- a/lib/pages/settings.dart
+++ b/lib/pages/settings.dart
@@ -9,6 +9,7 @@ import '../link/net.dart';
 import '../shell_scope.dart';
 import '../theme/light.dart';
 import '../theme/palette.dart';
+import '../theme/sun.dart';
 
 class SettingsPage extends StatelessWidget {
   const SettingsPage({
@@ -58,14 +59,20 @@ class SettingsPage extends StatelessWidget {
               title: const Text('Dark or bright'),
               subtitle: ValueListenableBuilder(
                 valueListenable: themes.lux,
-                builder: (context, lux, _) => Text(
-                  lux == null
-                      ? "Auto follows Android's dark theme and its schedule (no light sensor). "
-                            'The status bar button holds one; a long press there is auto again.'
-                      : 'Auto follows the light sensor: dark at ${LightSwitch.darkAt.round()} lux or less, '
-                            'bright from ${LightSwitch.brightAt.round()}; now ${lux.toStringAsFixed(lux < 10 ? 1 : 0)} lux. '
-                            'The status bar button holds one; a long press there is auto again.',
-                ),
+                builder: (context, lux, _) {
+                  final sun = themes.sunElevationNow;
+                  final now = lux != null
+                      ? 'now ${lux.toStringAsFixed(lux < 10 ? 1 : 0)} lux; dark at '
+                            '${LightSwitch.darkAt.round()} or less, bright from ${LightSwitch.brightAt.round()}'
+                      : sun != null
+                      ? 'now ${sun.round()}° above the horizon at the last GNSS position; '
+                            'dark below ${darkBelowDeg.round()}°'
+                      : 'no GNSS position yet';
+                  return Text(
+                    'Auto follows ${themes.autoSource} ($now). '
+                    'The status bar button holds one; a long press there is auto again.',
+                  );
+                },
               ),
             ),
             Padding(
diff --git a/lib/theme/palette.dart b/lib/theme/palette.dart
index 2aea9ee..67e6e91 100644
--- a/lib/theme/palette.dart
+++ b/lib/theme/palette.dart
@@ -24,6 +24,7 @@ import 'package:flutter/services.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import 'light.dart';
+import 'sun.dart';
 
 /// The HUD face (smart-serow's DIN 1451). Opt-in: a widget names it in its own
 /// TextStyle; everything else keeps the basic font, which covers CJK where DIN does not.
@@ -120,21 +121,36 @@ class Palette {
   );
 }
 
-/// Which variant shows: auto (the light sensor, else Android's dark theme), or one held fixed.
+/// Which variant shows: auto (a light sensor, else the sun, else Android's dark theme),
+/// or one held fixed.
 enum VariantMode { auto, dark, bright }
 
 /// A theme: both variants, and which one is showing. Listen to it to follow a switch.
-/// In auto it follows the tablet's light sensor ([followLight]); without one, Android's
-/// dark theme (by hand or on its schedule). The mode is kept across restarts.
+/// In auto it follows a light sensor ([followLight]) where the device lets an app read
+/// one; else the sun at the car's last position ([setPosition]); else Android's dark
+/// theme (by hand or on its schedule). The mode and the position are kept across restarts.
 class Themes extends ChangeNotifier with WidgetsBindingObserver {
-  Themes(this.name, this.dark, this.bright, [this._prefs]) {
+  Themes(
+    this.name,
+    this.dark,
+    this.bright, [
+    this._prefs,
+    this._clock = DateTime.now,
+  ]) {
     _mode =
         VariantMode.values.asNameMap()[_prefs?.getString(_key)] ??
         VariantMode.auto;
+    final (lat, lon) = (_prefs?.getDouble(_latKey), _prefs?.getDouble(_lonKey));
+    if (lat != null && lon != null) _position = (lat, lon);
+    _sunDark = _sunNow();
+    _sunTimer = Timer.periodic(const Duration(minutes: 1), (_) => _sunCheck());
     WidgetsBinding.instance.addObserver(this);
   }
 
   static const _key = 'variant_mode';
+  static const _latKey = 'sun_lat';
+  static const _lonKey = 'sun_lon';
+  final DateTime Function() _clock;
 
   final String name;
   final Palette dark;
@@ -157,7 +173,7 @@ class Themes extends ChangeNotifier with WidgetsBindingObserver {
   /// The last light level, in lux, while a sensor reports (for Settings, to tune by).
   final lux = ValueNotifier<double?>(null);
 
-  /// Follow a light sensor in auto. Errors (no sensor) leave auto on Android's dark theme.
+  /// Follow a light sensor in auto. Errors (no sensor, or refused) leave auto to the sun.
   void followLight(Stream<double> levels) {
     void changed(bool? before) {
       if (_light.isDark != before && _mode == VariantMode.auto) {
@@ -190,9 +206,51 @@ class Themes extends ChangeNotifier with WidgetsBindingObserver {
     );
   }
 
+  (double, double)? _position;
+  bool? _sunDark;
+  late final Timer _sunTimer;
+
+  /// The car's position, for the sun. Kept (to 0.1°, about 10 km: plenty for the sun)
+  /// so auto knows the sun before the first fix.
+  void setPosition(double lat, double lon) {
+    double r(double x) => (x * 10).round() / 10;
+    final p = (r(lat), r(lon));
+    if (p == _position) return;
+    _position = p;
+    _prefs?.setDouble(_latKey, p.$1);
+    _prefs?.setDouble(_lonKey, p.$2);
+    _sunCheck();
+  }
+
+  /// The sun's elevation now, in degrees, or null with no position yet.
+  double? get sunElevationNow {
+    final p = _position;
+    return p == null ? null : sunElevation(p.$1, p.$2, _clock());
+  }
+
+  bool? _sunNow() {
+    final e = sunElevationNow;
+    return e == null ? null : e < darkBelowDeg;
+  }
+
+  void _sunCheck() {
+    final now = _sunNow();
+    if (now == _sunDark) return;
+    _sunDark = now;
+    if (_mode == VariantMode.auto && _light.isDark == null) notifyListeners();
+  }
+
+  /// What auto follows now, in words, for Settings.
+  String get autoSource => _light.isDark != null
+      ? 'the light sensor'
+      : _sunDark != null
+      ? 'the sun'
+      : "Android's dark theme";
+
   bool get isDark => switch (_mode) {
     VariantMode.auto =>
       _light.isDark ??
+          _sunDark ??
           WidgetsBinding.instance.platformDispatcher.platformBrightness ==
               Brightness.dark,
     VariantMode.dark => true,
@@ -206,11 +264,16 @@ class Themes extends ChangeNotifier with WidgetsBindingObserver {
 
   @override
   void didChangePlatformBrightness() {
-    if (_mode == VariantMode.auto && _light.isDark == null) notifyListeners();
+    if (_mode == VariantMode.auto &&
+        _light.isDark == null &&
+        _sunDark == null) {
+      notifyListeners();
+    }
   }
 
   @override
   void dispose() {
+    _sunTimer.cancel();
     WidgetsBinding.instance.removeObserver(this);
     _holdTimer?.cancel();
     lux.dispose();