If you want better mpg, buy a vacuum gauge, and watch it and learn it, don't waste your money with closed loop, it's ace at protecting cats but a correctly mapped car doesn't need it, but you won't have optimum fuelling from an off the shelf chip, if you could get this there would be no point in live mapping as there would be no gain to be had!!
I don't run CL and can get 40mpg without problem, and that was on a 200,000 miles worn engine, correctly mapped CO was 0.97% on greys and 137ppm.
tabetha