Gear class from Sandi Metz's "The Magic Tricks of Testing"

gear.rb

class Gear
  attr_reader :chainring, :cog, :wheel
  def initialize(args)
    # ...
  end

  def gear_inches
    ratio * wheel.diameter
  end

  private
  def ratio
    chainring / cog.to_f
  end
  # ...
end