You are absolutely correct on the tank capacity. The turbo 700 series do not do that well on gas mileage. We own a fair market sampling of turbo cars, and the M46 versions have got as high as 26.5 mpg on the freeway, but 23-25 mpg is more common for freeway use in the speed range which you stated. In town the turbo cars get 20-22 mpg with M46. The automatic transmission cars have never got over 22 mpg on the freeway, and 19-20 mpg is pretty common in town. All fuel that goes into our cars is written in a log carried in each car. Ownership of turbo 700 series goes back six years.