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Lugging is actually good for fuel economy because you're running higher throttle opening and lowering pumping loss.
My bet is that 5th is significantly better for cruising.
If you can't borrow a scangauge to confirm, and you have a consistent driving pattern, try a tankfull with 5th, then one with 4th, then one with 5th and one with 4th. If one gear is clearly better, that should show it.
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