I regularly get 400 miles to a tank, not particularly easy, mostly local runs 10/20 miles.
NOT closed loop on greys, T3/34, I suppose on £20 150 easy, driving normally, which is foot down with me, as said didn't buy it to go slow.
Best thing you can do is avoid chips in favour of individual mapping, whilst chips are ok etc do the job, real benefits are seen on most with mapping work, esp in the mpg dept when driving easy.
tabetha