3_40.rb

# When Gear is part of an external interface
module SomeFramework
  class Gear
    attr_reader :chainring, :cog, :wheel
    def initialize(chainring, cog, wheel)
      @chainring = chainring
      @cog       = cog
      @wheel     = wheel
    end
    # ...
    def gear_inches
      ratio * wheel.diameter
    end

    def ratio
      chainring / cog.to_f
    end
  end
end

class Wheel
  attr_reader :rim, :tire
  def initialize(rim, tire)
    @rim       = rim
    @tire      = tire
  end

  def diameter
    rim + (tire * 2)
  end
end

# wrap the interface to protect yourself from changes
module GearWrapper
  def self.gear(chainring:, cog:, wheel:)
    SomeFramework::Gear.new(chainring, cog, wheel)
  end
end

# Now you can create a new Gear using keyword arguments
puts GearWrapper.gear(
  chainring: 52,
  cog:       11,
  wheel:     Wheel.new(26, 1.5)).gear_inches
# => 137.0909090909091