I reading this and I do not understand why there is an issue with fuel consumption.
You should remember that in both the turbo and superchargers' cases they aim to increase the efficiency of the engine. However, a turbo only gets to that once it has spooled up. A supercharger is always there. UNLESS ...
---In modern supercharged engines the blower is by-passed when not needed (low throttle posotions). When you push-on an electrically operated pulley (like the one on your ac) is activated and the charger spooles into action. I have never been a fan of this setup because of its complexity and prefer the "always-on" type.---
This brings me back to my previous point, the efficiency and the consumption.
When the efficiency is increased your engine hp is increased. Now, where you previously needed to depress your throttle say 50% to maintain 70mph you'll now only need to depress it say 30% to get the same 70mph. Oviously the calc's are not as simple as that but you get the picture.
So the decision you are left with is, "How do I get a better gear ratio?"
In VW they used a lot of generic parts so we used to mix & match for a certain ratio. I am new to Alfa ownership so I cannot say if we can do the same.
But why not look at some of the diesel boxes and maybe and size or two up or down on the tyres to get the ratio you'd like (+ the appropriate calibration of the speedo).
Let me just say, I love your idea.
I wonder if somebody out there has done this to a diesel powered car (any car for starters)?
Maybe I should start a thread on that!!