Run your car only on V-Power. Obviously half a tank of petrol weighs less than a full tank so you are dragging less weight around. As V-Power starts to take effect, you will find you can run the engine at lower revs in the gears and this is how V-Power can improve economy. Don't labour the engine. Just change up earlier and don't change down as readily. You'll find the car goes round corners in third that previously needed second. It will not improve economy if you simply drive the car in the normal way.