From my memory of the first posts in this article - from some of our best educated people - adding a turbo by itself does not guarantee increased FE. To get the benefit, you will need either a smaller engine or taller gearing, and probably both.
Assuming the same engine, you'll get a wider power band, starting at much lower rpms. You'll also get more fuel into the engine at any given rpm/throttle combination. In fact, that's the purpose of a turbo - to get more fuel into the same size engine, thus more power out. European auto makers used turbos to get around displacement limitations in some countries - most notably in Italy. Italy had a 2 liter maximum for many years, larger engines had a huge jump in taxes/fees. So they sold 1.999 liter turbo'd engines in Italy, made by several Euro manufacturers (Volvo for one, with the
"Polar" 240).
Going back to the earliest posts, I think that to get a FE benefit from a turbo you will need either a smaller engine or taller gearing, and probably both. When you consider the cost of the turbo + installation effort, I dunno that it's worth it. You might do much better with just maybe one size bigger tire (taller, not wider), a lighter foot, and a good FE gauge like an MPGuino.
Then there's the issue raised by bkrell in post #101 above. One of several really - but with the turbo in play, you're pressing the whole system to the max possible, stressing the oil, creating more blowby, etc etc.
If you want to find an engine that's about 1/2 the size of the original and turbo it, you might get improved FE. If you found one 2/3 to 3/4 size you could just install it with decent gearing without a turbo, and have a car that gets good FE.
Just my .02