Logic: car makes less effort if both engine and turbo work together.
Rule: Turbo needs to have compression, introduced at certain revs.
Logic: car needs more fuel at higher revs
Rule: sure, but, if you're gaining speed you want to make the multiplying factor the higher possible, so get there fast and then use a higher shift to cruise.
Logic: Lower revs save fuel
Rule: Only, and only if you're at a higher rev (depending on shift there's a minimum stable speed) and crusing - keeping the speed with minimum pedal effort.
What really gives is that you should "surf" the torque, try to keep the engine compressed. But keep in mind that only a higher shift can save the mileage.
Why don't you try it for a while?
(A sample of CB instant will tell you that, fuel is pumped to achieve the 2000 mark, but between this and 3000 you see a reduction on the consumption, and shifting up makes this better)
I have a skydome, you get lot's of picture on this thread:
Chipped windscreen
PL