Exactly Fido. I've always hated it when manufacturers publish "range" numbers. The more important number is endurance. How long can I keep it in the air at a given speed and land with a certain reserve. Wind speeds can vary so widely at the altitudes flown that "range" is a totally relative number.
As for a straight G-IV, I'm looking for a place to land at about 8:00-8:15 at long-range cruise. In a G-IVSP, maybe 8:30-8:45.
I have flown several different G4's and have found that each will vary in range and endurance to some degree. Just putting a loaner engine one can effect the endurance as much as 20 minutes. Loaner/Rental engines tend to be weaker and burn more fuel than your own. Also LRC will add around 20 to 30 minutes of endurance with little effect on speed. Depending on weight, altitude and OAT, It will generally run between .78 and .79 in LRC. With the fuel burns about 200 lbs a side lower.
I've flown several 8:50+ legs, landing with 45min of fuel in a -SP. This was usually after a westbound crossing, where we departed Europe with less than max fuel (~28.7K, due to specific gravity blend differences in Jet-A throughout Europe) and at a constant mach (usually M.80). If you can truly take advantage of long range cruise (which is a moving mach number, usually starting somewhere around M.82 and slowing to M.77 or so late in the flight) AND have a full 29.5K of gas, you should be able to do 09+00 and land with VFR reserves (NOT NBAA).
Some will say that for best fuel economy the autothrottles should be disengaged at cruise and power managed manually. If your A/T's are operating properly, I've found that it doesn't makes any real difference.
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