The comments werent so much good gas mileage cant be done, just that his mention of going to a 6 spd, which seemed to include numerically lower gearing in 6th, for getting better mileage werent going to work, because you'll just end up in an rshared_pm range where the engine isnt as efficient, and consume more fuel.

There is a reason nissan put the gearing where they did. Sure .5:1 overdrive and 3.4x gears work great on a V8, where that puts you where its most efficient on fuel, but on a little 4 cyl, thats generally not the case.