[The_Jed]

I think there's a bit of a parabolic curve for the returns realized by taller gearing. Sure, at a steady state lower rpms will usually equal better f.e. but, the effort required to accelerate to that speed or maintain it against aerodynamic drag will be increased with the taller gears.

I think there's a compromise "sweet spot" for gearing that will yield the best overall f.e. while not necessarily the best in any one set of conditions. That being said, something may be wrong. Your car is 800-1,000 lbs lighter than mine and I have no overdrive yet the worst tank f.e. EVER for my car was about 13 mpg and that was with lots of playing in the snow.

You could build on very potent and efficient Fox with junkyard parts! I could write my parts list (I've done TONS of research over the years) if you'd like.