diff --git a/lib/quantity/systems/si.rb b/lib/quantity/systems/si.rb
index 61de52c..7ac3f2c 100644
--- a/lib/quantity/systems/si.rb
+++ b/lib/quantity/systems/si.rb
@@ -24,7 +24,7 @@
 # @see http://physics.nist.gov/cuu/Units/prefixes.html
 class Quantity
   class Unit
-    module SI
+
   prefixes = {}
   units = {}
   aliases = {}
@@ -68,7 +68,7 @@ module SI
 
   units.each do | unit, dimension |
     prefixes.each do | prefix, value |
-      add_unit dimension, "#{prefix + unit}".to_sym, value, "#{prefix + unit}s".to_sym
+      add_unit "#{prefix + unit}".to_sym, dimension, value, "#{prefix + unit}s".to_sym
       if aliases[unit]
         aliases[unit].each do | unit_alias |
           add_alias "#{prefix + unit}".to_sym, "#{prefix + unit_alias}".to_sym
@@ -81,7 +81,7 @@ module SI
   add_alias :centimeter, :cm
   add_alias :meter, :m
   add_alias :nanometer, :nm
-  add_unit :length, :angstrom, 10 ** -7, :angstroms
+  add_unit :angstrom, :length, 10 ** -7, :angstroms
 
   add_alias :kilogram, :kg
   add_alias :gram, :g
@@ -89,16 +89,13 @@ module SI
   add_alias :megagram, :tonne, :tonnes
 
 
-  #l = Unit.for(:length)
-  Quantity::Dimension::Compound.name_compound self.for(:meter).dimension**3, :volume
   prefixes.each do | prefix, value |
-    add_unit :volume, "#{prefix}liter".to_sym, value * 1000, "#{prefix}liters".to_sym
+    add_unit "#{prefix}liter".to_sym, :volume, value * 1000, "#{prefix}liters".to_sym
     (aliases['liter']).each do | unit_alias |
       add_alias "#{prefix}liter".to_sym, "#{prefix + unit_alias}".to_sym
     end
   end
   add_alias :liter, :l
 
-    end
   end
 end
diff --git a/lib/quantity/systems/us.rb b/lib/quantity/systems/us.rb
index eb8b548..b061b79 100644
--- a/lib/quantity/systems/us.rb
+++ b/lib/quantity/systems/us.rb
@@ -3,17 +3,17 @@
 # fluid pint and a dry pint, pleases see the documentation
 # for the quantity/systems/us modules.
 class Quantity::Unit
-  add_unit :length, :foot, 304.8, :ft, :feet
-  add_unit :width, :inch, 25.4, :in, :inches
-  add_unit :length, :yard, 914.4, :yd, :yards
-  add_unit :length, :mile, 1_609_344, :miles
+  add_unit :foot, :length, 304.8, :ft, :feet
+  add_unit :inch, :width, 25.4, :in, :inches
+  add_unit :yard, :length, 914.4, :yd, :yards
+  add_unit :mile, :length, 1_609_344, :miles
 
-  add_unit :mass, :pound, 453592.37, :pounds, :lb, :lbs
-  add_unit :mass, :ounce, 28349.5231, :ounces, :oz
-  add_unit :mass, :ton, 907184740, :tons
+  add_unit :pound, :mass, 453592.37, :pounds, :lb, :lbs
+  add_unit :ounce, :mass, 28349.5231, :ounces, :oz
+  add_unit :ton, :mass, 907184740, :tons
 
-  #add_unit :fluid_ounce, 29.57, :floz, :ozfl
-  #add_unit :pint, 473.18, :pint, :pints
-  #add_unit :quart, 946.35, :qt, :quarts
-  #add_unit :gallon, 3785.41, :gallons, :gal
+  add_unit :fluid_ounce, :volume, 29.57, :floz, :ozfl
+  add_unit :pint, :volume, 473.18, :pint, :pints
+  add_unit :quart, :volume, 946.35, :qt, :quarts
+  add_unit :gallon, :volume, 3785.41, :gallons, :gal
 end
diff --git a/lib/quantity/unit.rb b/lib/quantity/unit.rb
index 756de26..7523952 100644
--- a/lib/quantity/unit.rb
+++ b/lib/quantity/unit.rb
@@ -58,7 +58,7 @@ def self.add_alias(unit,*names)
     # @param [Numeric] value
     # @param [[String Symbol]] *aliases
     def self.add_unit(name,dimension,value,*names)
-      new_unit = Unit.new({ :name => name,:dimension => dimension,:value => value})
+      new_unit = Unit.new({ :name => name,:dimension => Quantity::Dimension.for(dimension),:value => value})
       names.each do | name |
         add_alias new_unit, name
       end
@@ -242,7 +242,7 @@ def self.from_string_form(to)
     def initialize(opts)
       @units = opts[:units]
       @dimension = opts[:dimension]
-      @value = @dimension.is_base? ? opts[:value] : calculate_value
+      @value = opts[:value] || calculate_value
       if @dimension.nil?
         raise ArgumentError, "Adding invalid unit with nil dimension (#{name} - #{dimension})"
       end
diff --git a/spec/unit.spec b/spec/unit.spec
index 2fb1e54..b771735 100644
--- a/spec/unit.spec
+++ b/spec/unit.spec
@@ -43,10 +43,14 @@ describe Quantity::Unit do
     Quantity::Unit.add_unit :gram, @mass, 1000, :grams
     Quantity::Unit.add_unit :nanosecond, @time, 10**-6, :nanoseconds
     Quantity::Unit.add_unit :picogram, @mass, 10**-9, :picograms
+    Quantity::Unit.add_unit :mps, @accel, 10**12, :meterspersecond
     meters = Quantity::Unit.for :meter 
     meters.dimension.should == @length
     meters.name.should == :meter
     meters.value.should == 1000
+    mps = Quantity::Unit.for :mps
+    mps.name.should == :mps
+    mps.value.should == 10**12
   end
 
   it "should know its aliases" do