The only way that turbos increase MPG is by decreasing the engine size. Car manufactures use turbos in the cars to create the same amount of horsepower with a smaller displacement engine.
For example, BMW 335 uses twin turbos to enable a 3.0 liter engine to output around 300bhp. Without turbos, the same engine would probably require a 3.5 liter to make the same power. At idle and cruising, a 3.0 liter engine will use less gas compared to the 3.5, therefore you "increase MPG by turbos".
However, slapping on turbos on the same engine with same displacement will NOT increase MPG.
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