chips/programmers in gassers can add mileage if you drive right. they adjust fuel and ignition timing to move the torque curve to more along where you drive.
chips/programmers in diesels will adjust the fuel timing and injection pressure. typically, this gets you marginal on emissions standards.
getting one for performance. it will help. getting one for fuel mileage is marginal at best and can hurt because you drive more to the power available.