Quote:
Originally Posted by Daox
If you don't downsize the engine the best you can hope for is the same mileage which is highly unlikely.
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That's not accurate.
There are other benefits than just being allowed to reduce engine size. Your intake charge needs to be compressed before ignition. This is typically done in the cylinder by the piston. If you compress it with a supercharger, cool it down, and then compress again with the piston, you can compress the intake charge more efficiently.
As mentioned earlier, a supercharger can allow you to use lower gear ratios than NA which can improve fuel efficiency as well.
Most cars have superchargers added for performance reasons, so looking at empirical data won't help you much. However, if the engineers design for efficiency only, it's entirely reasonable for a supercharger to increase mileage.
Mike