I don't think it's a tuning or engine issue. From the drag coefficients listed, it is probable that the poor higway mileage is attributable to that. The car's drag force increases proportionally to the square of the velocity, so this becomes a more dominant factor at highway speeds than on the carerra-like contour, explaining why the Zetec gets better mileage at low speeds but suffers at high speeds.
The shape of the car does not lend itself well to highway economies.
My two cents
