+1, my old man told me that 5th should be reserved for anything over 80, and that goes for the majority of cars
you need a certain amount of revs to cycle the oil system to the required pressure, I would never drive 60kph in 5th unless it was downhill (in which case the drivetrain is not under load)... the gear ratios in a GTR are quite nice imho, I wouldnt want to change the ratios for a daily driven car... on my weekend hills run, i very rarely used 5th gear and the fastest we went was 115kph, so I kept it in 4th for most of the way, and the entire 2 hour run took just a tad over a quarter tank... most of the time the car was in 3rd doing 4000rpm around corners, so I wasn't being gentle, and yet it took much less fuel than I estimated
-D