diff --git a/src/Sound/fg_fx.hxx b/src/Sound/fg_fx.hxx
index e03ffcb49..15b7de961 100644
--- a/src/Sound/fg_fx.hxx
+++ b/src/Sound/fg_fx.hxx
@@ -68,6 +68,7 @@ private:
   double _old_gear_position;
 
   bool _gear_on_ground[MAX_GEAR];
+  float _wheel_spin[MAX_GEAR];
 
 				// looped sounds
   FGSimpleSound * _engine[MAX_ENGINES];