Another fill, this time 57.7 mpg / 669 miles.
Highway mileage is a challenge. As best as I can find, the Fit's drag coefficient is 0.33 or 0.34 (Civic 0.32). Being a taller car, it also has a higher frontal area than the Civic (+9%). That's a combined increase of 16% aerodynamic drag. Then there are the gear ratios.