Is it true that Super Chargers increase the HWY Gas Mileage ?

Well, in theory, they can. During WWII Charles Lindbergh was a civilian adviser in the Pacific. He found that the range of the P-38 could be increased by using higher levels of boost and lower RPMs. The pilots thought that it would cause engine temperature to get too high and cause the engines to fail. But it worked. Of course that's an airplane and not a car.