gistfile1.rb

class CarConfig
  include ActiveSupport::Configurable
  config_accessor :brand
end

class Car
  attr_accessor :brand
  
  def initialize
    self.brand = self.class.config.brand
  end
  
  class << self
    attr_reader :config
  
    def configure
      @config ||= CarConfig.new
      yield @config if block_given? 
    end
  end
end

Car.configure do |config|
  config.brand = 'Volvo'
end