diff --git a/lib/core_ext/big_decimal.rb b/lib/core_ext/big_decimal.rb
index 2f88fc6..959ea18 100644
--- a/lib/core_ext/big_decimal.rb
+++ b/lib/core_ext/big_decimal.rb
@@ -1,9 +1,14 @@
-require 'corrency/corrency_mixin'
+require 'corrency/methods'
 
 class BigDecimal
-  include CorrencyMixin
+  include Corrency::Methods
 
-  def to_s(s = "%.2f")
-    sprintf(s, self)
+  old_to_s = instance_method(:to_s)
+  define_method(:to_s) do |s = nil|
+    if s.nil?
+      sprintf('%.2f', self)
+    else
+      old_to_s.bind(self).call(s)
+    end
   end
 end
diff --git a/lib/core_ext/fixnum.rb b/lib/core_ext/fixnum.rb
index c6cafd9..56882d9 100644
--- a/lib/core_ext/fixnum.rb
+++ b/lib/core_ext/fixnum.rb
@@ -1,5 +1,5 @@
-require 'corrency/corrency_mixin'
+require 'corrency/methods'
 
 class Fixnum
-  include CorrencyMixin
+  include Corrency::Methods
 end
diff --git a/lib/corrency/corrency_mixin.rb b/lib/corrency/corrency_mixin.rb
deleted file mode 100644
index 7f4f73c..0000000
--- a/lib/corrency/corrency_mixin.rb
+++ /dev/null
@@ -1,30 +0,0 @@
-require 'bigdecimal'
-require 'bigdecimal/util'
-
-module CorrencyMixin
-  def inc_vat
-    (self.to_d * vat_rate)
-  end
-
-  def ex_vat
-    (self.to_d / vat_rate)
-  end
-
-  def next_ninety_nine
-    self.ceil - BigDecimal.new("0.01")
-  end
-
-  def add_margin(margin)
-    ((self.to_d / (100 - margin.to_d)) * 100).round(2)
-  end
-
-  def margin(cost)
-    self_d = self.to_d
-    ((self_d - BigDecimal.new(cost)) / self_d) * 100
-  end
-
-  protected
-    def vat_rate
-      1 + (Corrency::Config.vat_rate.to_d / 100)
-    end
-end
diff --git a/lib/corrency/methods.rb b/lib/corrency/methods.rb
new file mode 100644
index 0000000..7d694a7
--- /dev/null
+++ b/lib/corrency/methods.rb
@@ -0,0 +1,32 @@
+require 'bigdecimal'
+require 'bigdecimal/util'
+
+module Corrency
+  module Methods
+    def inc_vat
+      (self.to_d * vat_rate)
+    end
+
+    def ex_vat
+      (self.to_d / vat_rate)
+    end
+
+    def next_ninety_nine
+      self.ceil - BigDecimal.new("0.01")
+    end
+
+    def add_margin(margin)
+      ((self.to_d / (100 - margin.to_d)) * 100).round(2)
+    end
+
+    def margin(cost)
+      self_d = self.to_d
+      ((self_d - BigDecimal.new(cost)) / self_d) * 100
+    end
+
+    protected
+      def vat_rate
+        1 + (Corrency::Config.vat_rate.to_d / 100)
+      end
+  end
+end
diff --git a/test/big_decimal_test.rb b/test/big_decimal_test.rb
index d876219..b43971c 100644
--- a/test/big_decimal_test.rb
+++ b/test/big_decimal_test.rb
@@ -18,7 +18,7 @@
     end
 
     it "should accept a format override" do
-      assert_equal '31.4159', BigDecimal.new('31.4159265358979').to_s('%.4f')
+      assert_equal '+314 15.926 53', BigDecimal.new('31415.92653').to_s('+3F')
     end
   end
 end