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Take all reports of mileage w/a grain of salt, especially radically low ones (like 20mpg hiway) which usually mean a problem with maintenance or foot weight.
That being said, turbo works by forcing more air into the manifold, which needs to be married with an appropriate amount of fuel. More air, more fuel, which is why we see frequent reports of ~27mpg hiway for normally aspirated 4-banger Volvos and maybe 24mpg for turbo units of same displacement. Which all neatly matches up w/improved horsepower and torque of turbo, percentage-wise.
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