Since 6th gear is an overdrive (hence the OD in the display) it is purely there to reduce engine rpm at cruising speeds.
Overdrives have a small effect on fuel economy but it's mostly about reducing wear on the motor/gearbox.
Fuel economy is almost all down to drag once yer running at constant speeds.
Higher speed = higher drag.
Someone on here did a speed run on an airfield a few years back (on a faster 1st Gen,.. naturally!
) and clocked 176Mph with no side bags and OEM screen, but I can't remember if he'd removed the 'Flies and fitted a Power Commander.
That was in 5th gear.
My personal record was 165Mph indicated (in 5th, probably about 155Mph real) on the Autobahn (A3, Frankfurt to Bonn) whilst "playing" with a guy on a ZX12R. We had some fun!