a

class Gear
  attr_reader :chainring, :cog, :rim, :tire
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog       = cog
    @rim       = rim
    @tire      = tire
  end

  def gear_inches
    ratio * Wheel.new(rim, tire).diameter
  end

  def ratio
    chainring / cog.to_f
  end

end

# ~~~~~~

puts Gear.new(52, 11, 26, 1.5).gear_inches
puts Gear.new(52, 11, Wheel.new(26, 1.5)).gear_inches

def gear_inches
  #... a few lines of scary math
  foo = some_intermediate_result * diameter
  #... more lines of scary math
end

  def wheel
    @wheel ||= Wheel.new(rim, tire)
  end