of course a turbo increases efficiency. just not fuel economy by itself.
put a turbo motor into a car designed for non turbo, and change NOTHING else, and you will get worse or same economy.
now, take that car, and give it a taller geared differential, and do other things to take advantage of the better horsepower, and you have a better mpg vehicle.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!