I own a Skyline, a starlet turbo, and a Glanza turbo (auto)
Believe me fuel is an increasing problem as i use v max on all 3.
Point being, both the Starlet and Glanza have identical engines, one a manual the other auto, but i get a lot more miles with the auto, purely because of the way i drive them, both feel like they always want to go faster.
However with the auto, i just relax and drive reasonably normal and fuel consumption is at least 25% better.
Obviously i cant tell how you drive, but i should imagine with a Cooper driving it like it should be you are not being the most economical, check your revs not the speedo
All the experts say you should accelerate and change to higher gear asap to be most economical ( economical does not apply to 600bhp Skylines whatever you do :>) and it has been proven that if you use V Max sensibly it will give the same if not more MPG, and no i dont work for Shell....