The venerable 2cv (citroen) although only a 2 cyl engine, got in
excess of 50mpg, and could cruise at 65 all day in its later
iterations. Their full size sedans such as the ID 19, while providing
ample seating room for 5 adults also got in excess of 35 mpg (as
measured by me in daily use in the 70's). So it is possible to get
decent mileage and comfort...it was done almost 40 years ago.