I think either a taller 5th or 6th would be fine for better fuel economy. The point isn't higher top speed, but lower cruising RPM, which would save fuel. If you want acceleration or power for climbing, you downshift.
I've put a super-tall final drive in my ~50 hp Firefly/Metro that no engineer would have ever specified (the f.d. from the 4-cylinder transmission in the 3-cylinder car). Yes, acceleration suffered because it affects all gears (not just top gear), but top gear cruising fuel economy improved, and that was my goal.